we can easily identified that each box is a full subtractor.
where Y is difference bit and Z is Borrow bit.
if A,B,C is a Inputs to Full-Subtractor, and we are doing A-B
Difference = A ⊕ B ⊕ C
Borrow = A' . B + A' .C + BC
But we have to identify is it doing P-Q or Q-P ?
By seeing Z = R . Q + P'. R + P' . Q = P'. R + P' . Q + R . Q ===> we can say it is P - Q
Option B is right