What type of functional group do aldehydes contain in their structure?

Aldehydes are characterized by the presence of a carbonyl group, which consists of a carbon atom double-bonded to an oxygen atom (C=O). In aldehydes, this carbonyl group is located at the terminal end of the carbon chain, which distinguishes it from other compounds that may also contain carbonyl groups, such as ketones, where the carbonyl group is found within the carbon chain.

The defining feature of aldehydes is not only the carbonyl functional group but also that it is bonded to at least one hydrogen atom, making them distinct in nature and reactivity. This structural arrangement significantly influences their chemical properties and behavior in reactions.

Therefore, the correct answer focuses on the carbonyl group as the key functional group that defines aldehydes, highlighting its importance in the class of organic compounds.