12) F

It is important for you to realise that the diameter of the circle is the diagonal of the rectangle.

Let width of rectangle be W. So length is 3W

2(3W + W) = 24 cm
8W = 24 cm
W = 3 cm
Length = 3 x 3 = 9 cm

Diagonal = hypotenuse of one right-angled triangle.
Diagonal2 = 32 + 92
Diagonal2 = 9 + 81
Diagonal2 = 90
Diagonal = 3√10
Radius = (3√10 ÷ 2) cm

Area of circle = π (3√10 ÷ 2) 2 = 90/cm2
Area of rectangle = 9 x 3 = 27 cm2
Area of shaded area = (90/4) π – 27 cm2 

Obviously, this is not one of the options. However, F is the same as the answer. So F is correct.