As the rule stated, a semicolon is prefered rather than a comma if you have two independent clauses connected without a conjunction. If you have two long independent clauses when conjunctions such as and, or, but, for, nor, you will prefer a comma.
So, the correct option is D. He couldn't find his baseball glove; it was under a pile of clothes.
Since we have two independent clauses without a conjuction to connect them, a semicolon is employed to separate them.
