Answer
Aldehydes, Ketones, Carboxylic acids, Acid Chlorides, Amides and Esters are six classes of organic compounds that include the group $-CO-$ in some form.
Work Step by Step
R = Alkyl Groups (for example $-CH_{3}$, $-CH_{2}-CH_{3}$ etc. groups)
For aldehyde, if the R group is replaced by H, it is the simple aldehyde i.e. formaldehyde.
For Carboxylic acid, if the R group is replaced by H, it is formic acid.
For ester group, the R bonded with C can be H or any alkyl groups. But the R attached with R should be only alkyl groups not H. Also, two R groups of ester may be same or different.
