Hi preeti, the given scenario is not a valid scenario.I am 
explaining you in deatil.The Grade respective to the score 
will not valiadte.It will be like this

1)90-100---A
2)80-89----B
3)70-79----C
4)Else-----F

OR it can be 91-100----A
             81-90-----B
             70-80-----C
             Else---F  
And the boundary value analysis for this Object(0-100)
1)Min(0)-----Pass(0)F Grade
2)Max(100)---Pass(100)A Grade
3)min(+1)----Pass(1)F Grade
4)Max(-1)----Pass(99)A Grade
5)Min(-1)----Fail(-1)F Grade
6)Max(+1)----Fail(101)F Grade

for each validation user should have to write 6 test cases
Total validation above mentined 4 condtions so, (4*6)=24 
test cases will occur.
For example considering first case:90-100(for grade A)

1)Min(90)-----Pass(90)A Grade
2)Max(100)--- Pass(100)A Grade
3)min(+1)---- Pass(91)A Grade
4)Max(+1)---- Fail(101)F Grade
5)Min(-1)---- Fail(89)B Grade
6)Max(-1)---- Pass(99)A Grade

Higher level scenerios : 

If the score is 90 to 100 - grade A
If the score is 80 to 89 - grade B
If the score is 70 to 79 - grade C
If the score is 0 to 69 - grade F

your answer was correct only.

Hi Madhu,

You have left the one condition.You have consider the 
conditin for score falls between 0-70 but not considered 
the case, if score is greater than 100.

so it means the q was tricky, he was looking that I will 
point out these things or not as a tester. 
But these are all BVA right.We can check functionality 
also. Is it correct or not?

yaa qestion was tricky,as u r saying we can check 
functionality its fine we can apply BVA for Text box but 
here system will fail at the value 80 and 90.For this 
scenario atleast two bug will definetly occure at the value 
80 and 90.

The answer given by Santosh Kumar is correct.
PLease include that for each and every case u need to check 
the go button action also.

Good explanation !! almost the same question was asked to 
me but it was about three combo fields "country","state" 
& "city" and I was asked to create the scenarios. I replied 
but without folllowing any methodology. Can you help me to 
explain those scenario?

Hi Preethi,

checking BVA is secondary, first we need to find out the
High Level Test case. what he is asking..

So here we can say one test case. which is functionality
test case, 
when user clicks on the Go button, weather it is giving
write message(grade) or not,
and i want to say one more thing for all, for checking BVA
we need Low Level Document(mandatory), then only we will go
for BVA. without having LLD, don't even think about the BVA.

Hi Preeti whenever you come across such questions in
interview we need to draw a flowchat for the asked scenario
and then decide how many High level test cases can we come
across, we have some topics which covers how many test cases
can be written for each and every flow , this is not an
ordinary question this is the question which they ask for
certification level. 

Here is the flow for your answer


               _________               
               | start |
               |_______|
                   |
               _________
               | Score |
               |_______|
                   |
                   / \
                  /   \      Yes        ________
                 / 90- \  _____________ | "A"   |___
                 \100  /                |_______|   |
                  \   /                             |
                   \ /                              | 
                    |                               |
                    |  No                           |  
                    |                               |
                   / \                              |
                  /   \     Yes          ________   |
                 / 80- \  _____________ | "B"   |___|
                 \ 90  /                |_______|   |
                  \   /                             |
                   \ /                              | 
                    |                               |
                    |No                             | 
                    |                               |
                   / \                              |
                  /   \                  ________   |    
                 / 70- \  Yes__________ | "C"   |___|
                 \ 80  /                |_______|   |
                  \   /                             |
                   \ /                              |
                    |                               | 
                    |No                             |
                ____|____                           |
               |  "F"   |                           | 
               |________|                           |
                    |                               |
                ____|____                           |
               |  End   |___________________________| 
               |________| 

The above flowgraph explains the scenario which you have said
Now obverse that from this scenario we can see that if
90-100 condition pass we get Grade "A" 
if 80-90 condition pass we get Grade "B"
if 70-80 condition pass we get Grade "C"
else we can see "F".

So if we check then there wil be exactly "4" High level test
cases. I need to leave now I will explain in other time
continuine this answer.

Answer given is right. Only thing is approach. Now What does
High level test cases means. If U clerify and is High
priority then u should go with Boundary value.
Again different approaches are there. If Interviewer wants
-ve test cases (As for tester high level means test cases
which will cause system unfunctional).