Answer:
C. -CH3
Step-by-step explanation:
The pKa (acid dissociation constant) of an amino acid residue is primarily influenced by the functional groups attached to the adjacent end of the amino acid chain. The acidity of a functional group depends on how electronegative the functional group is. In the given functional groups' list, the functional group with the least influence on the pKa of the amino acid is the alkyl substituent (-CH3), because they have the least electron-withdrawing capacity.