An enantiomerically pure acid is treated with racemic mixture of an alcohol having one chiral carbon. The ester formed will be

When an optically pure acid is made to react with racemic mixture of an alcohol (d or l form), it results in the formation of two types of isomeric esters as shown in the example below. Here, the configuration of the chiral carbon remains unchanged or we can say same. Thus, the mixture will be optically active. Thus, the correct option is (a) optically active mixture