Answer: A subject, a verb, and a complete thought.
Exaplnation: A simple sentence is a sentence that contains one independent clause. An independent clause is a group of words that expresses a complete thought and can stand alone as a sentence. A simple sentence must have a subject, a verb, and a complete thought. The subject is the "who" or "what" the sentence is about, and the verb is the action that the subject performs. The complete thought is the meaning of the sentence.
For example, the sentence "The dog barked" is a simple sentence because it has a subject (the dog), a verb (barked), and a complete thought (the dog made a barking sound). The sentence "The dog barked and the cat ran away" is not a simple sentence because it contains two independent clauses, each of which expresses a complete thought.
The other answer choices are incorrect. A simple sentence does not need a conjunction, and it does not need a complete thought if it is only a fragment. The "who" or "what" the sentence is about is the subject, not the complete thought.