Option D is the correct answer because “back out” means to withdraw from a commitment, which is correct here because singing the agreement is a commitment that can not be broken.
Option A is incorrect because “back up” means an extra emergency source, which is not related to commitment and agreement here.
Option B is incorrect because “back in” means to enter the agreement again, statement says that he cannot enter in again, which is incorrect because the statement mainly mentions the condition of the agreement is being broken.
Option C is incorrect because “back at” is usually followed by an “it” to state that somebody is back at doing or performing a particular task.