<p>@zxm123
Answer is D. </p>
<p>Area of semi-circle with diameter of AC is half the area of the corresponding circle or pi(AC)^2/8, and similarly area of semi-circle with diameter of AB is pi(AB)^2/8. Sum of the areas is pi[AC^2 + AB^2]/8. Also from Pythagorean theorem: AB^2 + AC^2 = k^2, therefore the area is equal to pi(k^2)/8. </p>