Answer:
C. Heat must be added or removed
Explanation:
When matter changes states it is due to the overall loss or gain of heat. In addition, heat being added or removed is what will cause a temperature change, but a simple temperature change is not enough to change states. For example, a temperature could go from 50 degrees to 51. This will not cause a change in states. So heat must be added or removed in order for matter to change states.