Choice B is the best answer. There are two independent clauses in the sentence, each with a subject and a verb: "many...were skeptical" and "historians have determined…." These clauses can be grammatically joined by a comma and the coordinating conjunction "but."

Choice A is incorrect. This choice results in a run-on sentence, which occurs when two independent clauses are joined without punctuation. Two independent clauses can’t be joined by just the coordinating conjunction "but." A comma would also be required. Choice C is incorrect. This choice creates a punctuation error called a comma splice. This sentence contains two independent clauses ("Many…were skeptical" and "historians have determined…"). A comma alone can’t join two independent clauses. That requires a comma and a coordinating conjunction. Choice D is incorrect. This choice results in a run-on sentence, which occurs when two independent clauses are joined without punctuation. This sentence contains two independent clauses ("Many…were skeptical" and "historians have determined…"), which need to be either joined by a semicolon, joined by a comma and a coordinating conjunction, or separated by a period.