Which reagent distinguishes aldehydes from ketones by giving a positive (silver mirror) reaction only with aldehydes?

**Tollens' reagent** is ammoniacal silver nitrate: $[\mathrm{Ag(NH_3)_2}]^+$. Aldehydes reduce $\mathrm{Ag^+}$ to metallic silver, depositing the characteristic **silver mirror** on the inside of the test tube. Ketones don't reduce it. **Why the others fail the distinction.** - Fehling's (A) also distinguishes aldehydes from ketones (brick-red Cu₂O ppt with aldehydes) — but the question asks for the *silver mirror* test specifically, which is Tollens'. - 2,4-DNP (C) reacts with *both* aldehydes and ketones (it tests for the carbonyl group, period), so it can't tell them apart. - Br₂/CCl₄ (D) is the alkene/alkyne test, not a carbonyl test.