Which word group is a complete sentence? almost all the swimmers following the rules
never had any interesting activities
after chris finished the chemistry experiment
only two of the chairs mathed the table

The correct answer is D. Only two of the chairs matched the table.

Explanation

A sentence is a set of words that expresses an idea with complete meaning and syntactic autonomy. The sentence is composed of a subject, verb, and in some cases other elements that complete the meaning.

According to the above, the sentences "A. Almost all the swimmers following the rules" and "B. Never had any interesting activities" do not have a verb or express a complete idea, nor do they have a subject. On the other hand, the option "C. after Chris finished the chemistry experiment " is not a complete idea. However, in the case of the last option "D. Only two of the chairs matched the table" the sentence is complete as it contains a subject (two of the chairs), the verb (matched) and is a complete idea. So, the correct answer is D. Only two of the chairs matched the table.

the correct answer would be:  

a. adverb phrase