Which of the following amino acid is optically inactive?

Glycine

The answer is (4) glycine.

Glycine is the only amino acid that is optically inactive. This is because it has no chiral center. A chiral center is a carbon atom that is bonded to four different groups. Glycine has only two hydrogen atoms bonded to its α-carbon, so it does not have a chiral center.

The other options are incorrect. Phenyl amine, asparagine, and glutamic acid all have chiral centers, so they are optically active.