Answer:
A. when their ship docked
Explanation:
A dependent clause is a group of words with a subject and a verb. However, it cannot stand alone as it does not express a complete thought. B expresses a complete thought and can stand alone as a sentence, so we can eliminate that as a possible answer. Although C can seem like a possible answer, A is a better choice because it features the dependent marker "when".
I believe it's
B. Were swimming
The answer would be true the phrase alone can be a sentence