Which best explains why the diagram shows refraction but not reflection?

There's no way to tell without seeing the diagram.

The answer would be that it shows that the ray is changing direction. Refraction will occur when light changes direction due to the object it is passing through. For example, as a lifeguard, when looking for submerged victims, they may appear distorted to the naked eye through the water because the light is "refracted" because of the light passing through the water's surface.