Answer:
A. No change needs to be made to this sentence
Step-by-step explanation:
Both parts of this sentence are independent clauses that can function separately. We can easily say, "We have fifteen minutes to complete the tests." and "Howard's class has another hour." as two separate sentences. This means that a semi-colon is an appropriate punctuation between the two independent clauses. Replacing the semi-colon with a comma would create a run-on sentence. All other punctuation is correct.