please give ratio for M25to M40. because the ratio
available in IS code only M10-M20, most case M25,so more
then M25 ratio is our own preparation. so my suggestion is
M20 or M25.

M40
because its have more cement content and it also depends
upon what is mix design if you are giving cement content in
M20 more than M40,then M20 will give higher strength than
M40

M40 becoz the ratio of cement:sand:aggregate is higher
then the other. which are the core factor for any mix. the
property of any mix like compreesive strength,
characterstic strength etc . largely depends on the ratio.

what is the factor of safety for cement and steel with
reference to IS 456-2000. and min reinforcement should be
provided for column and slab with reference to IS 456-2000.