Answer:
C). Although most of my friends go to public school.
Explanation:
A Subordinate clause is described as an independent clause including a subject and verb and beginning with either a relative pronoun(that, which, who) or a subordinate conjunction (like although, if, After, since, etc). Such clauses do not offer a complete thought and remain dependent on the principal to clause to finish or complete the thought.
As per the question, the entire subordinate clause in the given sentence would be 'Although most of my friends go to public school' as it begins with subordinate conjunction 'although' and dependent on the principal clause to complete its meaning(justifies the above claim). Thus, option C is the correct answer as the other three offer incomplete ones.
