A circle has radius 8. What is the area of a sector with a central angle of 90°?

The sector is $\frac{90}{360}=\frac{1}{4}$ of the circle. The area of the circle is $\pi(8)^2=64\pi$, so the sector's area is $\frac{1}{4}\cdot64\pi=16\pi$, choice C. Choice D ($32\pi$) uses a half circle, the sector for $180^\circ$. Choice B ($8\pi$) uses an eighth. Choice A ($4\pi$) uses a sixteenth. A $90^\circ$ central angle is a quarter turn, so the sector is always one quarter of the circle's area, whatever the radius. Using fractions of the circle avoids needing a calculator.