Which of the following group show +M > –I effect?
The oxygen of the ester group is in conjugation with the carbonyl of the −COR group. The carbonyl group is electron withdrawing and this rules out the aldehyde and carboxylic acid groups as unsuitable as per the question.
The oxygen of the −O−CO−R is relatively free to conjugate its one of the two lone pairs. This is despite the fact that the O−CO bond has double bond character.