Explanation

[Solution from source] CH₃CHO contains a CH₃CO– group and gives a yellow iodoform precipitate with I₂/NaOH, while C₆H₅CH₂CHO has no methyl ketone/aldehyde group and does not. Both are aldehydes with α-hydrogen, so both reduce Tollens' reagent and (being aliphatic-type aldehydes) Fehling's and Benedict's solutions, which therefore cannot distinguish them. Answer (3).