* Temperature and Molecular Motion: Temperature is a measure of the average kinetic energy of the molecules within a substance. Higher temperatures mean molecules move faster and further apart. Lower temperatures mean molecules move slower and closer together.

* Contraction: As the temperature decreases, the molecules slow down and their average distance from each other decreases. This results in the substance occupying a smaller volume, which is known as contraction.

Let's look at the other options:

* A. Most substances will expand: This is the opposite of what happens. Expansion occurs when temperature increases.

* B. gain atoms: Decreasing temperature doesn't change the number of atoms in a substance.

* D. d? This option is incomplete and doesn't make sense in the context of the question.