virtual function is used to overddin that function in 
derived class

The Virtual is a keyword it used in the base class.When we
use the Virtual keyword in the function it must be override
in the  inherited class with the same name.

If the instance function declared with a virtual modifier 
then the function is called virtual function.

the derived class can give the redefinition to the virtual 
function its optional.

Second answer given here is correct to some extent.
If we use Virtual keywork next to a mehtod,it can be 
overridden in derived classes,but it is not must to 
override.

Virtual Method are feature of object oriented programming
and used to Identify the Type of object that is calling any
particular method at the Run time so provides flexibility in
calling method.

when we override a metod in sub class,and we want to hide 
the decelaration of the base class member then we used the 
keyword virtual in superclass,
and keyword override in the subclass

intersection of 4 & 6

If a base class method is to be overriden, It is defined 
using virtual keyword. You need to use the override keyword 
in order to re-implement the virtual method. Exp:

public class Employee
{
public virtual void SetBasic(float money) //This method may 
be overriden
{ Basic += money; }
}


public class Manager : Employee
{
public override void SetBasic(float money) //This method is 
being overriden
{
float managerIncentive = 10000;
base.SetSalary(money + managerIncentive); //Calling base 
class method
}
}