Answer: B) which were swimming near the boat.
Step-by-step explanation: An adjective clause is a dependent clause (a phrase that doesn't have complete meaning on its own) that acts like an adjective, this means that it modifies a noun or pronoun. Adjective clauses begin with words such as that, when, where, who, whom, whose, which, and why. In the given sentence we can see an example of an adjective clause in the phrase "which were swimming near the boat" this clause modifies the noun "whales."