This question tests the understanding of the characteristics of a main clause. The correct answer is that a main clause must have a subject and a verb, making it independent and able to stand alone as a sentence. Option A is incorrect because inversion is a feature of certain types of questions or sentences, not main clauses in general. Option B is incorrect because a main clause does not necessarily contain a relative pronoun. Option C is incorrect because a main clause can indeed stand alone as a sentence.