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.
