Answer:
a). does not make sense on its own
Step-by-step explanation:
A subordinate clause is characterized as the clause that is dependent on the principal clause to accomplish its meaning. Such clauses usually begin with a subordinating conjunction or relative pronoun and function to offer additional information in the sentence.
In the given complex sentence, the emphasized group of words ('although...basketball') exemplifies a subordinate clause as it 'does not convey a complete sense on its own' and require the principal clause('he prefers...baseball) to accomplish its meaning. However, it contains a subject and verb but cannot stand on its own. Thus, option a is the correct answer.