What is the measure of each exterior angle of a regular octagon?

The exterior angles of any convex polygon sum to $360^\circ$. A regular octagon has $8$ equal exterior angles, so each measures $\frac{360^\circ}{8}=45^\circ$, choice C. Check using the interior angle: $\frac{(8-2)180^\circ}{8}=135^\circ$, and $180^\circ-135^\circ=45^\circ$. Choice B ($40^\circ$) is $\frac{360^\circ}{9}$, the exterior angle of a nine-sided polygon. Choice A ($36^\circ$) is $\frac{360^\circ}{10}$, for a decagon. Choice D ($60^\circ$) is $\frac{360^\circ}{6}$, for a hexagon. Count the sides of the polygon carefully. Also, the interior and exterior angles at each vertex add to a straight angle.