Question

Do you understand what they are asking?

Which of the following physical properties can be used to measure temperature using a thermometer?
volume
pressure
density
viscosity

Im guessing they are talking about viscosity but I don't quite fully understand it

Answer 1

Think about the thermometer. It is a sealed capillary tube with mercury or color alcohol inside. When the bulb is heated, the liquid expands, and the only place it has to go is up the tube. The pressure inside is the same, the density and viscosity (over a few degrees, at least) are essentially the same so what's left?
http://en.wikipedia.org/wiki/Thermometer