What is an aldehyde?

An aldehyde is characterized by the presence of a carbonyl group (C=O) that is positioned at the terminal end of a carbon chain. This unique placement of the carbonyl group distinguishes aldehydes from other carbonyl-containing compounds, such as ketones, where the carbonyl group is found within the carbon chain. Therefore, describing an aldehyde as a structure where the carbon skeleton has the carbonyl functional group located at the end highlights its defining feature. This terminal position contributes to the distinctive chemical properties and reactivity of aldehydes, making them important intermediates in organic synthesis and relevant in various biological processes.

In this context, it's important to recognize the roles of the other options: while some compounds may contain functional groups or different carbon skeleton structures, they do not aptly define aldehydes, which are specifically tied to the terminal carbonyl position.