The correct option is A A group of words that contain a subject and a verb, forming a complete meaning.
A sentence is made up of a subject and a verb, these are crucial to a sentence. Without these, the group of words cannot be called a sentence. For example, 'The moon rises', where the subject is the moon and rises is the verb. From the options, B describes the sentence, hence it's correct. Option A describes a dependent clause which is not a sentence and option C describes a phrase. Hence options A and C are incorrect. Option D is incorrect because the answer lies in B.