Draw a triangle on your paper and extend the base of the triangle so it extends past the base angle. The base angle inside the triangle is the interior angle and the one outside the triangle between the extended line and the outside of the interior angle is the exterior angle. A straight line has a measure of 180 degrees. That means that the interior angle + the exterior angle = 180. So they are supplementary by definition of supplementary angles. A is the only choice above that applies.
