Is the answer a or b and why?

Answer:
B
Step-by-step explanation:
The area between the curves y = d and y = f(x), between the limits x=a and x=b, is:
∫ₐᵇ (d − f(x)) dx
Alternatively, the area between the curves x = f(y) and x = a, between the limits y = c and y = d, is:
∫ᵈ (f(y) − a) dy
Answer A would give you the area between x = f(y) and the y-axis. It needs to include the − a term to be correct.