Which of the following undergoes Haloform reaction?

The haloform reaction is given by compounds containing a methyl ketone group ($\text{CH}_3\text{-CO-}$) or compounds that can be oxidized to this group, such as alcohols containing the $\text{CH}_3\text{-CH(OH)-}$ group.

Among the given options, $(\text{CH}_3)_2\text{CO}$ (acetone) contains the $\text{CH}_3\text{-CO-}$ group and therefore gives a positive haloform test.

$\text{HCHO}$ (formaldehyde) lacks the $\text{CH}_3\text{-CO-}$ group.

$\text{C}_2\text{H}_5\text{Cl}$ and $\text{CH}_3\text{-O-CH}_3$ do not possess the required structural features to undergo the haloform reaction.

Answer: $(\text{CH}_3)_2\text{CO}$