The author’s word choice makes the tone clear __ bitingly sarcastic.

The correct answer is A.

Explanation: The use of a colon after “clear” indicates that what follows is an explanation or elaboration of the tone. In this case, “bitingly sarcastic” serves to clarify the type of tone the author employs. A semicolon would incorrectly suggest a connection between two closely related independent clauses, which isn’t the case here.