d Glycine is the only amino acid that is not optically active because it does not have a carbon chiral center. In order for a carbon to to be a chiral center, it must have 4 different R-groups attached to it. Gylcine has only three different $R$ groups on it (an amine group, a carboxyl group, and two $H$ 's), which means it does not have a chiral center, and is also not optically active.
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