Both carboxylic acids and esters are organic compounds with C=O bonds, however carboxylic acids contain very polar O-H groups which can hydrogen bond while esters do not. Therefore carboxylic acids have generally relatively high boiling points and are therefore not very volatile, while esters are. Additionally, esters tend to have strong aromas, and are used in many flavoured and scented products, while carboxylic acids are not. So the answer is D, esters.
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