Which of the following is a simple sentence?

A simple sentence consists of a single independent clause, which means it contains a subject and a verb and expresses a complete thought. In this case, the option that qualifies as a simple sentence is actually none of the ones provided, but let's clarify the details around choice C.

Choice C contains the subjects "The dog and the young, active cat" and combines two actions linked by "and." This structure clearly indicates multiple subjects and actions, moving it into the realm of a compound sentence.

To understand why other options also do not represent simple sentences, consider the remaining choices.

Choice A features a dependent clause "Once I have finished," which introduces the independent clause "I will stop by," making it a complex sentence.

Choice B presents two independent clauses joined by "and," which categorizes it as a compound sentence.

Choice D also combines two independent clauses with "but," making it another compound sentence.

In summary, none of the choices provided are simple sentences since all contain additional clauses or multiple subjects and verbs. A simple sentence should not contain any conjunctions or additional phrases that create complexities. For effective identification of simple sentences, look for a single subject and predicate that conveys a clear, standalone idea.