Answer:
D
Explanation:
The sentence in answer A is a comma splice. You cannot separate two independent clauses with only a comma.
The sentence in answer B is a run-on sentence.
The sentence in answer C is also a run-on sentence.
This leaves only answer choice D, which is the correct answer since it combines two independent clauses with a coordinating conjunction.
(a coordinating conjunction is a word that links two independent clauses such as for, and, nor, but, so, etc.)
