1. Each N atom is reduced from 0 to 3.
Each H atom is oxidized from 0 to +1.
2. Each N atom is reduced from 0 to −6.
Each H atom is oxidized from 0 to +2
3. Each N atom is reduced from +1 to −3.
Each H atom is oxidized from +1 to 0.
4. Each N atom is oxidized from 0 to −6.
Each H atom is reduced from 0 to +2.
5. Each N atom is reduced from 0 to +3.
Each H atom is oxidized from 0 to −1.
6. Each N atom is oxidized from 0 to +3.
Each H atom is reduced from 0 to −1.
7. Each N atom is oxidized from 0 to −3.
Each H atom is reduced from 0 to +1.
8. Each N atom is oxidized from +1 to +3.
Each H atom is reduced from +1 to 0.
1 Answer

The N in N2 and H in H2 are both zero, so that eliminates some. In NH3, the N is 3 and H is +1, so the answer is #1.