Ok, first of all, this is in the wrong subject, but I'll answer it anyways.
4NH3 is 4 molecules of NH3, which is ammonia.
So the 3 in NH3 would be the subscript since there are three hydrogen molecules in one ammonia molecule.
5O2 is 5 molecules of O2, which is oxygen.
The subscript would be 2 since there are 2 oxygen molecules in oxygen.
6H2O is 6 molecules of H2O, which is water.
The subscript would be 2 since there are 2 hydrogen molecules in water.
So the subscripts are 3, 2, 2 or answer choices A and B.
