The correct answer is A. The squirrel, who ran in circles around the circumference of the tree, chattered angrily at the curious cat.
Explanation:
This sentence is a complex one, which means it is composed by an independent clause, which is a sentence that express a complete idea and a dependent clause, which is a type of sentence that provides extra information linked to a independent clause and depends on it to be complete, as dependent clauses are just extra information and can be omitted they are always marked by commas (,) or similar punctuation marks. In this case, the main sentence is "The squirrel chattered angrily at the curious cat" as it expresses the main idea, on the other hand "who ran in circles around the circumference of the three" is just extra information (dependent clause) and thus should be between commas. Thus, A is the correct revision as this sentence contains a dependent clause that according to grammar should be between commas.
