Step 1: Recall the reagent behaviour
Cold dilute neutral KMnO$_4$ makes diols, but acidified KMnO$_4$ cleaves the double bond.
Step 2: Product
Cleavage gives carbonyl fragments, and acidic KMnO$_4$ oxidises any aldehyde to RCOOH. So the product class is alkanoic acid, option (D).
Final Answer:
Acidified KMnO$_4$ oxidises alkenes to alkanoic acids, option (D).
\[ \boxed{\text{Alkanoic acid}} \]