The correct answer is: A) he fears the monster will attack Clerval and kill him.
Indeed, at this point in the plot, the Creature has already killed Frankenstein’s younger brother William. He has also revealed his presence to Frankenstein and demanded that he make him a female creature wife. Frankenstein has accepted but has later changed his mind, worried that both creature will be evil and be a scourge for humanity. The Creature has threatened to kill his family and friends should he not comply.
Frankenstein still refuses, therefore the Creature does kill Henry Clerval and leaves his body and frames Frankenstein as the murderer.