2003 AMC 10B Problem 23

Let $O$ be the center. The octagon splits into $8$ congruent triangles from $O,$ so $\triangle AOB$ has area $\dfrac18.$ Since $O$ is the midpoint of $AE,$ triangles $AOB$ and $BOE$ have equal area, so $\triangle ABE$ has area $\dfrac14.$ The rectangle $ABEF$ is twice this, namely $\dfrac12.$ Thus, the correct answer is D.