The molecules briefly form chemical bonds with nearby objects. The molecules are always in random motion. The molecules transfer an electric charge to the dust particles. The molecules frequently change shape.
I gave my students this multiple choice question: When observing a very small piece of dust in a liquid under a microscope, the dust appears to jiggle slightly. This movement results from the dust interacting with the molecules in the liquid. What property of the molecules in the liquid causes this effect?

Only one answer is correct among these 4 choices:
- The molecules briefly form chemical bonds with nearby objects.
- The molecules are always in random motion.
- The molecules transfer an electric charge to the dust particles.
- The molecules frequently change shape.

Could you tell me which one is correct?

The molecules are always in random motion.