C. The motion of the atoms should vary depending on the state.
In solid bromine, the atoms are closely packed in a fixed arrangement and vibrate in place. In liquid bromine, the atoms are still close but can move past each other, allowing the liquid to flow. In gaseous bromine, the atoms are far apart and move freely and rapidly. The different states of matter are characterized by the motion of the atoms, so this is the key difference in her models.