A: Main clause.
Independent (main) clauses are essentially clauses that can stand on their own. They do not depend on another clause's existence to make sense as a statement/question. "So many legends surround the history of the United States flag." There, that's a sentence. If someone burst into the room right as you said that, they would not assume they had missed any part of the statement. If the fact that it is a sentence starting with so, don't be, because in this case so acts as an adverb describing how many, not as a conjunction, and so as a conjunction is one of the coordinating conjunctions that introduces independent clauses anyways.
Dependent (subordinate) clauses need another clause in the sentence in order to exist. They depend on the main clause happening ignorer to happen. They often, but not always, begin with a word that indicates for you that it is dependent. "That" is such a word. "That" can serve different purposes depending on how it is used, but if it does not work like the word "this" would, then it is probably introducing some form of depended clause. "That it is sometimes difficult to establish the the true story." Not a complete sentence—just read it aloud, on its own, if you think otherwise. Some other words that can introduce a dependent clause include whenever, although, and because.
It gets a bit trickier if you have a sentence containing a dependent clause not indicated by a word such as that. Fortunately, that's not the case here, so that's another headache for another day.