Final answer:
The side chain or R group is the functional part of an amino acid in the active site. Arginine is an amino acid with a functional side chain that is likely to be found in the active site of an enzyme.
Step-by-step explanation:
The functional part of this amino acid in the active site is the side chain or R group. The side chain, also known as the functional group, is the unique part of an amino acid that distinguishes it from another amino acid. It plays a crucial role in determining the structure and function of the protein. In this case, the side chain of the amino acid is the part that interacts with other molecules in the active site of an enzyme.
In terms of the options given, (b) Arginine is the amino acid with the functional part in the active site. The side chain of arginine contains a guanidinium group, which can form various interactions such as hydrogen bonding, salt bridges, and electrostatic interactions with other molecules in the active site.