The answer is D. On both sides of the equation, there are 8 C, 16 O, 24 S.
We can also do this by elimination.
For A, the left side has 2 (NO3) but the right side has 3 (NO3).
For B, the left side has 3 C but the right side has 1 C.
For C, the left side has 2 Fe but the right side has 1 Fe