Which compound contains a carbonyl group?

The correct choice is B, as an aldehyde is characterized by the presence of a carbonyl group, which is a functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O). In aldehydes, this carbonyl group is located at the end of the carbon chain, giving them distinct properties and reactivity.

Alcohols, aromatic hydrocarbons, and alkanes do not contain a carbonyl group. Alcohols feature a hydroxyl group (-OH) instead, which defines their behavior and properties. Aromatic hydrocarbons are characterized by their ring structures with delocalized pi electrons, and do not contain a carbonyl group. Alkanes consist solely of carbon and hydrogen atoms linked by single bonds and lack both functional groups and the carbonyl structure.

This makes aldehydes unique within the options provided, showcasing their critical role in organic chemistry and highlighting why the presence of a carbonyl group is essential to their classification.