The correct answer is actually D), not A. Although these are independent clauses indeed, the punctuation in A,B, and C is not correct. Before 'however' there needs to be a semicolon (;). And after however, you have to put a comma. Therefore D is the correct answer. After a comma there needs to be a pause in reading. Does A sound correct to you if you make a pause before 'however' and not after it? Of course not.
D) Robert wanted a new car; however, he couldn't afford to buy one.
Explanation:
Two sentences can be joined with a semi-colon. The conjunctive adverb "however" must be followed by a comma. Robert wanted a new car; however, he couldn't afford to buy one. is correct.
