Question

which functional group is found in an ester?

Explanation:

Esters have the general formula R - COO - R'. The functional - group of an ester is the carboxylate group with the structure $\ce{R-COO - R'}$ (where R and R' are alkyl or aryl groups). The first option is a carbonyl group attached to two R groups (a ketone - like structure), the third option is an alcohol functional group, and the fourth option is an amine functional group.

Answer:

The second option $\ce{R - C(O)-OR'}$ (where the carbon is double - bonded to oxygen and single - bonded to an oxygen which is further bonded to an R' group) represents the functional group of an ester.