I believe the correct answer is the third option: Although
most of my friends go to public school.
Subordinate clause is the dependable clause, which
means that it can’t stand alone as it doesn’t express the full information.
These clauses are introduced by the subordinate conjunction or relative pronoun
(although, that, once, until etc.) and a comma. In this case, the subordinate
clause is adverbial clause (has a function of an adverb) “Although most of my
friends go to public school,” which is introduced with the subordinate
conjunction although and divided with a comma from the main clause as it is
inverted.
