The sentence with an adjective clause is C: Liz, who could always make people laugh, was loved by the whole block.
An adjective clause is a dependent clause that modifies a noun or pronoun. That means that they can't stand alone as a sentence. They usually begin with words such as <em>that, when, where, who, whom, whose, which</em>, and <em>why. </em>
In option C, "who could always make people laugh" is modifying Liz, it also starts with WHO and it can't stand alone as an independent sentence.