Adding or removing thermal energy to or from a substance does not always cause its temperature
to change.

a)true
b)false

Change in thermal energy not always cause it's temperature change. It is the situation when water reaches either at 0 C or 100 C then thermal energy doesn't cause change in temperature instead it changes the state of matter.

In short, Your Answer would be "True"

Hope this helps! 

Adding or removing thermal energy to or from a substance does not always cause its temperature
to change.

a)True

This is true in the case of boiling and melting point. That is when a change of state is involved.

For example in boiling the water gets to 100 degrees, if heat is continuously added, there is no change in temperature. The water simply changes state.