The statement that best describes and explains a character's actions is: T.J. cheats on the tests because Stacey will not answer his questions.
This statement captures the dynamic between the characters: T.J. is seeking help from Stacey regarding the tests, but Stacey is understandably upset because of the trouble he faced previously due to T.J.'s actions. It reflects Stacey's protective nature and his choice to draw a line against T.J.'s behavior, implying that T.J. resorts to cheating as a consequence of Stacey's refusal to help him.