Smoking’s an extremely harmful habit. You should give it up immediately.

Answer A

Explanation: There is a cause-and-effect relationship between the two sentences in the problem.

Smoking is an extremely harmful habit. You should give it up immediately.

Option D. As smoking is an extremely harmful habit, you should give it up immediately. = Since smoking is an extremely harmful habit, you should give it up immediately.

This is the meaning of the sentence that is closest to the meaning of the original sentence.