Once he has signed the agreement, he won’t be able to …… A. back up B. back in C. back at D. back out

The correct answer is D. back out.

To back out means to withdraw from something that you have agreed to do. In this case, the man will not be able to withdraw from the agreement once he has signed it.

The other options do not make sense in the context of the question. To back up means to move backwards, to back in means to drive into a parking space or driveway from the rear, and to back at means to criticize or scold someone.