Answer:
B. complex; Although most flash mobs gather to make a statement, some gather to protest something.
Explanation:
A complex sentence consists of an independent clause, which can stand on its own, and at least one subordinate clause, which does not express a complete thought. Besides, a dependent clause usually begins with a subordinate conjunction, such as <em>after before </em>and <em>because, </em>and is separated from the independent clause by a comma.
In the sentence given, the dependent clause is: <em>Although most flash mobs gather to make a statement. </em>Meanwhile, the independent clause is: <em>some gather to protest something.</em>
