The Hart-Dworkin Debate: A Conundrum

Arjun99 and I were studying Jurisprudence and we got into a heated discussion over this MCQ:

"In the context of Hart's Social Rules Theory, which one of the following is a characteristic of a valid legal rule?"

A) It is a moral rule B) It has a clear and certain standard C) It is a social rule D) It has been enacted by the government

Arjun99 says it's D) because Hart believes that legal rules are made by the government, whereas I say it's B) because Hart emphasizes that a valid legal rule has a clear and certain standard to ensure social cohesion and predictability. I'm dying here, guys - please settle this debate for us!