Acid/Base Chemistry with Amino Acids EK Question

I understand why the answer choice A is correct, but why isn't D correct also. If Arginine's side chain, when protonated, is more stable that means it's less likely to give off a proton and therefore has a higher pKa.

For instance, OH- has a high pKa because its conjugate acid (H2O) is stable.

Acid/Base Chemistry with Amino Acids EK Question