Q.An unknown nitro compound, on reduction with $\text{Sn}/\text{HCl}$, gives a primary aromatic amine that forms a stable diazonium salt at $0$–$5^\circ\text{C}$. What was the starting nitro compound, and what general class of reaction converted it to the amine?

Only a primary AROMATIC amine forms a diazonium salt stable enough to exist at $0$–$5^\circ\text{C}$ (Section 25.9/25.14) -- an aliphatic primary amine's diazonium salt decomposes instantly even in the cold. So the amine formed on reduction must have been aromatic, meaning the nitro compound reduced was an aromatic nitro compound -- nitrobenzene, reduced by a full reducing system such as $\text{Sn}/\text{HCl}$, $\text{Fe}/\text{HCl}$ or catalytic hydrogenation, to give aniline.

Work backwards from 'stable diazonium salt at $0$–$5^\circ\text{C}$' to 'must be an aromatic primary amine', then to 'must have come from an aromatic nitro compound'.

Suggesting an aliphatic nitro compound as the source -- its amine's diazonium salt would not be stable enough to observe at all.