C. The city, supposedly filled with gold, was a fable; and the tired explorers would never find it.
Option A has the error of a comma splice. The comma placed after “fable” is causing this error. A comma splice is when a comma is used when, in fact, a comma/conjunction, semicolon, or period should have been used. Option B is a run-on sentence. A run-on sentence is basically a sentence with two complete thoughts (independent clauses/complete sentences) that is missing the appropriate punctuation between them. In Option B, a comma should have been placed after “fable” in order to prevent a run on. Note the conjunction after “fable” in Option B is what will prevent a comma placed after “fable” from being a comma splice. Option C is correctly written because the semicolon is accurately placed between two independent clauses.