The subordinate clause in the sentence is option D: whereas Gordon preferred to play baseball.
A subordinaye clause is one which do not form a complete sentence, it depends on the other part of the sentence to have a total sense.
A subordinate clause, also known as a dependent clause, will begin with a subordinate conjunction or a relative pronoun and will contain both a subject and a verb.
In this case the word whereas indicates that it is a subordinate clause since it is a subordinate conjunctions.
