The option that makes the fragment a complete sentence is B. my cat plays with the yarn.
In this sentence, the first part <em>whenever I knit</em> is an <em>adverb clause</em> of time since it functions as an adverb that answers to the question when, and it is the <em>dependent clause</em>. The second part of the sentence is the <em>main clause </em>because it has a complete meaning and can stand alone.