Make sure to read and understand the question when taking these tests. The first sentence tells us that he is "going to the ground to complete the catch". Then it tells us that when he falls to the ground, the ball is in the end zone. By definition of a catch, it is therefore completed in the end zone. Touchback.
Going to the ground is relevant to
maintaining firm grasp and control of the ball, after having gained firm grasp and control with a body part down in the field of play. If firm grasp and control are maintained throughout the process of going to the ground, then the catch is completed, and progress is awarded to the most forward point of the ball during that catch process. In the case of the interception, the catch was, technically, completed at the B-1, when the interceptor returned to the ground with grasp and control of the ball. Technically, momentum would apply and Team B would get the ball at the B-1. HOWEVER, philosophically (OS 7-9) we should rule touchback when the spot of the completion
is in question as to being in the field of play or end zone. During my FBS days, that was always considered to be between the B-1 and the goal line. So, in this case, I would bet the correct answer will be Touchback.
As for Team A, the concept of progress also applies. If the catch is initially completed in the end zone (grasp/control of ball with body part down inbounds while ball is beyond/breaking the plane of the goal line, then Team A will get credited with a TD, even if the receiver falls to ground (or is driven back) in the field of play and maintains firm grasp and control of the ball.
I'm with Legacy in that, if they want a question that involves the standards, the questions should be more carefully composed. In this case, if they want us to consider OS 7-9, they should have said something like, "The covering officials could not positively verify that the interception was completed in the field of play or end zone."