Position and label the triangle on the coordinate plane. Select one.

The correct answer is the last one.
We know that the height is a. Since the height is given by the y coordinate, we want it to be a. So, the highest point of the triangle must be in the form [tex] (x,a) [/tex]. This is the case for options b and d.
Moreover, we know that the base must be 2b units long. The base of the triangle in option b starts at 0 and ends and b, along the x axis. This means that it is b units long, which is not what we want.
On the other hand, the half-base of the triangle in option d goes from 0 to b, which means that it it b units long. Since this is only half of the base, the complete base is 2b units long, which is what we want.