Which is most reactive towards esterification?
A.Formic acid.
B.Acetic acid.
C.$2-$methyl propanoic acid
D.$2,2-$dimethyl propanoic acid.

Hint: We know that it is a chemical reaction that takes place during the formation of the ester. Esterification is the chemical process that combines alcohol \[\left( ROH \right)\] and an organic acid \[\left( RCOOH \right)\] to form an ester \[(RCOOR)\] and water.

Complete step by step answer:
When carboxylic acids are heated with alcohol with the help of an acid catalyst, esters are produced. The catalyst in use is usually concentrated sulphuric acid. Alternatively, dry hydrogen chloride gas is useful in some cases which incline towards aromatic sweet-smelling esters, the ones with the benzene ring. The chemical reaction which occurs during the formation of the ester is known as the esterification.
When carboxylic acids are heated with alcohol with the help of an acid catalyst, esters are produced. The catalyst in use is usually concentrated sulphuric acid. Alternatively, dry hydrogen chloride gas is useful in some cases which incline towards aromatic sweet-smelling esters, the ones with the benzene ring. The chemical reaction which occurs during the formation of the ester is known as the esterification.
For esterification, the carbonyl carbon of the acid should be more electrophilic for more reactivity. Presence of +I groups like methyl group decrease this reactivity. Hence, \[HCOOH\] will be most reactive towards esterification.

Therefore, the correct answer is option A.

Note:
Remember that a functional group or a compound that is derived from the condensation of alcohol and acid with a simultaneous loss of water is known as an ester in chemistry. Carboxylic Easter which is also known as carboxylate ester or simply called ester is derived from carboxylic acid which is the most common form of Ester.