A black compound of manganese reacts with a halogen acid to give greenish-yellow gas. When an excess of this gas reacts with ${\mathrm{NH}}_{3}$ an unstable trihalide is formed. In this process, the oxidation state of nitrogen changes from-

1. -3 to +3

2. -3 to 0

3. -3 to +5

4. 0 to -3

Hint: In ammonia, the oxidation state of nitrogen is -3

Black coloured compound MnO2 reacts with HCI to produce greenish yellow coloured gas of Cl2
Clon further treatment with NH3 produces NCl3
NH3 (- 3) changes to NCl(+3) in the above reaction. Hence, (a) is the correct choice.