Which of the following is an aldehyde?

An aldehyde is a functional group that has the general formula CₓH₂ₓ₊₁CHO. The compound in option C is an aldehyde. Thus, option C is correct.
An aldehyde is a type of functional group that has a carbon atom that shares a double bond with an oxygen atom and another single bond with the R group and H group (R−CH=O).
The aldehyde functional group gives a specific chemical property to the species. The first compound is CH₃CH₂CH₂COOH is a carboxylic acid functional group. The second compound is CH₃CH₂CH₂COOCH₃ is an ester.
The third compound is CH₃CH₂CH₂CHO an aldehyde group as (R−CH=O) functional group is present. The fourth compound is CH₃CH₂CH₂COCH₃, ketone.
Therefore, option C is an aldehyde.
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