Which one of the following statements is correct?

The general structure of an α-amino acid is $$R{-}CH(NH_2)COOH$$. The central (α-) carbon will be chiral and therefore optically active if, and only if, it is bonded to four different groups.

For most naturally occurring α-amino acids the four groups attached to the α-carbon are:
1. $$NH_2$$ group
2. $$COOH$$ group
3. Side chain $$R$$ (different for each amino acid)
4. A hydrogen atom $$H$$

Because $$R$$ is different from $$H$$, these four groups are all different, giving a chiral centre and hence optical activity.

Glycine is the only exception. In glycine the side chain is $$R = H$$, so the α-carbon is attached to two identical groups (two hydrogens). Consequently, the α-carbon is achiral and glycine is optically inactive.

Now evaluate each option:

Option A - “All amino acids except lysine are optically active” is wrong; lysine has $$R = (CH_2)_4NH_2$$, giving four different groups around the α-carbon, so it is optically active.

Option B - “All amino acids are optically active” is wrong because glycine is not.

Option C - “All amino acids except glycine are optically active” is correct as explained above.

Option D - “All amino acids except glutamic acid are optically active” is wrong; glutamic acid has $$R = CH_2CH_2COOH$$, so it is optically active.

Hence, the correct choice is:
Option C which is: All amino acids except glycine are optically active.