The sentence that is correctly punctuated is, "Snack time had finally come, but no one had brought any snacks." The correct answer is option A. The sentence is an example of a compound sentence. A compound sentence consists of two independent clause. These clauses are connected either by a coordinating conjunction or a semicolon. But in this sentence, it is connected with a coordinating conjunction. The rule applied here is that, after the first clause, a comma should be applied before starting the next clause introduced by the conjunction.
