Final answer:
The compound CH₃CH₂CHO is an aldehyde, which is characterized by a carbonyl group bonded to at least one hydrogen and located at the end of the carbon chain.
Step-by-step explanation:
The compound that is an aldehyde is CH₃CH₂CHO, represented by option b). An aldehyde is characterized by having the carbonyl group (C=O) attached to at least one hydrogen atom and, in this case, it is at the end of the carbon chain. The functional group of an aldehyde can be represented as -CHO. Therefore, among the provided options, CH₃CH₂CHO is the only structure that fits the description of an aldehyde.