In a circle, a central angle measures 80°. What is the measure of an inscribed angle that intercepts the same arc?

The inscribed angle theorem says an inscribed angle is half the measure of the central angle that intercepts the same arc. The central angle is $80°$, so the inscribed angle is $\frac{80°}{2} = 40°$, choice B. Choice A ($80°$) confuses the inscribed angle with the central angle. Choice C ($160°$) doubles instead of halving. Choice D ($20°$) halves twice. A helpful memory: the vertex on the circle sees the same arc at a shallower angle than the center does, so the inscribed angle is always smaller than the central one.