Number of tectonic plates
This is primarily an exercise in interpreting a minimalist diagram. Believe me, you will have to learn to read even sketchier diagrams. 

That said, the test had the correct answer to the question. Unambiguously, there are only three plates.

The left and right plates each contain a piece of continental crust and a piece of oceanic crust connected by a passive continental margin. In each case, the oceanic terrane and the continental terrane move as a unit, so there's only one plate on the left and only one on the right. With the middle, oceanic-only plate, that makes three.

Although the diagram you gave us is a little more simplified than I'm comfortable with, you can tell that whoever drew the diagram meant for the left and right plates to contain passive margins. The elliptical loops of arrows are intended to be interpreted as convection cells in the mantle, and each cell drags the lithosphere above as a unit. So, anything above any one convection cell should be interpreted as one plate.

Here is a slightly more detailed diagram showing a passive margin:


(Source: Wikipedia, CC 3.0)