Acrolein is an α,β-unsaturated aldehyde

Steps:

• Deduce structure from C3H4O, DNPH orange precipitate, and Tollens' silver mirror: H2C=CHCHO.
• Confirm aldehyde functionality from given reactions.
• Check A for iodoform test applicability to structure.
• Check B for standard reduction of aldehyde group.
• Check C for alkene addition reaction.

Why C is correct:

• The C=C bond undergoes electrophilic addition with Br2, decolourising bromine water per the unsaturation test.

Why the others are wrong:

• A: Iodoform test requires CH3C(O)R or CH3CH(OH)R structure; acrolein has H2C=CHC(O)H, so no yellow CHI3 precipitate.
• B: α,β-Unsaturated aldehydes undergo conjugate (1,4) reduction with many agents, yielding propanal intermediate then propan-1-ol upon full reduction, but selective 1,2-reduction to allyl alcohol requires specific conditions; statement oversimplifies as "primary alcohol" without context.
• D: Not enough information.

Final answer: C