Final answer:
A clause is a group of words that contains both a subject and a verb. The group of words that is a clause is 'When he left the room'.
Step-by-step explanation:
A clause is a group of words that contains both a subject and a verb. It can be an independent clause, which can stand alone as a sentence, or a dependent clause, which cannot stand alone as a sentence. From the given options, the group of words that is a clause is A. When he left the room. This is a dependent clause because it starts with the subordinating word 'when' and relies on the main clause to complete its meaning.