Well, we know that the first one is correct because those are the only two shapes up there. The second one is correct because a*b*c is equal to the rectangular prism, while 1/2(a*b*[d-c]) is equal to the triangular one.
Keeping in mind that 1/2*d = c, the third one is also correct because when you simplify the equation, you get
This is equal to the volume because the overall height is 2, but since it's a triangular prism, we take away one half. Because it's all multiplication, we can actually arrange all the values. But the point is that the third one is right, too, leaving the last one to be wrong.
So the last one has to be the answer. Whew!