The correct option is B Someone's at the door. I heard somebody knock.
A compound sentence is made up of two independent clauses, which are
complete sentences on their own. Illative compound sentences contain
clauses where one is inferred or deduced from another; for example, "There's someone in the hall for the light is on", where "for" is an
Illative conjunction.
In this question, we need to choose the option that can use "for" as a conjunction. The sentence in option B. "Someone's at the door" is deduced from "I heard somebody knock". Hence B is the correct option. A contains sentences that contrast each other, it would require an adversative conjunction such as "I love chocolate but I don't eat cake" (since most cakes have chocolate). Hence option A is incorrect. Option C contains alternative sentences ("Neither do I smoke nor do I drink alcohol") in the negative. Hence C is also incorrect. Option D contains cumulative clauses ("Not only was I praised for my bravery but also given a medal for it"). Hence it's the incorrect option.