Describe one way that the student can determine the exact volume of one of the three blocks

Density is defined as an object's mass per a given unit of volume. So, if you know the object's density, and you're able to weigh it, you can determine its volume with the equation:

Volume = weight / density

There is a link in the Resources section to a web page which lists the densities of some common materials. Note that density changes with pressure or temperature.