If \(d\) = the diameter of the circle, then \(\frac{d}{2}\) = the radius of the circle.

Area of a circle \(= \pi r^2 = \pi (\frac{d}{2})^2 = \pi (\frac{d^2}{4}) = \frac{\pi d^2}{4} = a\)

So we now have:
QUANTITY A: \(\frac{\pi d^2}{2}\)

QUANTITY B: \(\frac{\pi d^2}{4}\)

If we don't already see that Quantity A is greater, we can always divide both quantities by \(\pi d^2\) to get:
QUANTITY A: \(\frac{1}{2}\)

QUANTITY B: \(\frac{1}{4}\)

Answer: A