Which functional group is present in compounds having the molecular formula $ {C_2}{H_4}{O_2} $ ?
(A) Carboxylic Acid
(B) Ester
(C) Amine
(D) Both A and B

Hint: First we have to look for the composition of given compound $ {C_2}{H_4}{O_2} $ , so it is formed by the combination of or addition of acetic acid and methyl methanoate. Now, we have to conclude the functional group of these two reactants.

Complete answer:
"Ester" and "carboxylic acid" are functional groups with two oxygen atoms. There are two functional isomers of the compound with the chemical formula $ {C_2}{H_4}{O_2} $ . Ethanoic acid or acetic acid (carboxylic acid) is one, and methyl methanoate is the other (ester).
Acetic acid, commonly known as ethanoic acid, is a colourless acidic liquid and organic substance having the chemical formula $ C{H_3}COOH $ (also written as $ C{H_3}C{O_2}H,{\text{ }}{C_2}{H_4}{O_2},{\text{ }}or{\text{ }}H{C_2}{H_3}{O_2} $ ). Apart from water, vinegar contains no less than $ 4\% $ acetic acid by volume, making acetic acid the most important component.
The methyl ester of formic acid is methyl formate, commonly known as methyl methanoate. It's a colourless liquid with an ethereal odour, high vapour pressure, and low surface tension, and it's the most basic example of an ester. It's a predecessor to a slew of other commercially valuable chemicals.
Hence, the correct option is (D) Both A and B.

Note:
In the presence of palladium catalysts, carboxylic acids and esters break down to yield alkenes and $ CO $ at temperatures exceeding $ 200{\text{ }}^\circ C $ . Acid anhydrides were decarboxylated with a variety of catalysts, with the reaction products varying greatly depending on the metal.