Which of the following amino acids is not optically active?

Glycine is the simplest and smallest amino acid. A compound shows optical activity, when a chiral carbon or asymmetrical carbon (ie, carbon which attached to four different groups or atoms) is present. Because chiral carbon is absent in glycine. So, it does not show optical activity. Other amino acids contain at least single asymmetric carbon atoms, so they show optical activity, eg, valine, leucine and isoleucine.