Which amino acid does not have a chiral α-carbon? 

Glycine 

The correct answer is option 3. Glycine.

Among the amino acids listed, glycine is the one that does not have a chiral \(\alpha \)-carbon.

A carbon atom is chiral if it has four different groups attached to it. In amino acids, the \(\alpha \)-carbon is the central carbon atom bonded to the amino group \((-NH_2)\), carboxyl group \((-COOH)\), hydrogen atom \((H)\), and a side chain (\(R\) group).

In glycine, the side chain is a hydrogen atom \((H)\). Since glycine's side chain is not a different group but just a hydrogen atom, the \(\alpha \)-carbon of glycine does not have four different groups attached to it. Therefore, glycine does not exhibit chirality at its \(\alpha \)-carbon. Alanine, valine, and leucine all have side chains (\(R\) groups) that are different from hydrogen, making their \(\alpha \)-carbons chiral.

Conclusion: Among the options given, glycine is the amino acid that does not have a chiral \(\alpha \)-carbon.

So, the correct answer is: Glycine