This is clearly a Safety if the penalty is accepted under NFHS rules. (The result is different under NCAA rules).
The Momentum Exception is off because the ball left the end zone. (8-5-2 Exception). The foul occurred during B37's run (which includes the loose ball) so the basic spot for enforcement is the end of the run and in the end zone. Under All but One, that will be the enforcement spot. Since the enforcment spot is in the end zone, it is a safety. If the penalty is declined, it will be B 1/10 @ B-6. But, A will certainly accept the penalty. 10-3-3-b, 10-4-4, 8-5-2-c
In NCAA, since the forward fumble OOB is returned to the spot where the run ended, it never left the end zone and the Momentum Exception would still be ON. In that case, the end of the run would be the momentum spot, the B-4. Under 3 and 1, the penalty would be enforced from there and it would be 1/10 for B ay the B-2.