Which of the following represents an aldehyde?

The correct answer is related to the structure of an aldehyde, which is characterized by the presence of a carbonyl group (C=O) attached to a hydrogen atom. In the choice identified, R-CH=O clearly illustrates this structure, where R represents any carbon-containing substituent, and the carbonyl group (C=O) is bonded directly to a hydrogen atom. This fulfills the definition of an aldehyde, where the carbonyl carbon is terminal (located at the end of a carbon chain).

The other choices do not fulfill the criteria for aldehydes. The first option, R-CO-R', represents a ketone since it has a carbonyl group flanked by two carbon-containing groups. The third option, R-C≡C-R', indicates an alkyne due to the triple bond between carbon atoms, while the last option, R-CH₂-CHO, depicts an aldehyde as well, but it is a more complex version with an additional carbon chain. The focus on the presence of a singular carbonyl terminal position makes the answer clearer, distinguishing it from the other functional groups represented in the other choices.