Typically, underlines, scribbles, and notes left in the margins by a former owner lower a book’s

Choice A is the best answer. The convention being tested is the coordination of independent clauses within a sentence. An independent clause is a phrase containing a subject and a verb that can stand on its own as a sentence. This choice uses a comma and the coordinating conjunction “but” to join the first independent clause (“underlines…lower a book’s value”) and the second independent clause (“such markings…can be a gold mine to scholars”) to create a compound sentence.

Choice B is incorrect because it results in a run-on sentence. The two independent clauses are fused without punctuation and/or a conjunction. Choice C is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between two independent clauses. Choice D is incorrect because a comma is needed to mark the boundary between two coordinated independent clauses.