Physics dynamics

the answer is D. can someone explain how to conclude that?


This is a tough one,
The force F moves the two blocks so F final = m+3m * a = 4ma
F/4m=a this is the acceleration of both blocks, but also the acceleration of block x and block y separately since they move from rest at the same speed and time
Fy = m * a = 3m * F/4m = 3F/4