Question

materials become less dense as their temperature is increased because
multiple choice
heat takes up space between molecules.
hot objects weigh less than cold objects.
their molecules expand.
increased energy results in molecules moving farther apart.

Explanation:

When the temperature of materials increases, the energy of their molecules increases. This increased energy causes the molecules to move more vigorously and move farther apart from each other. Density is calculated as mass divided by volume ($
ho=\frac{m}{V}$). As the volume ($V$) of the material increases (due to molecules moving apart) while the mass ($m$) remains constant (assuming no mass loss or gain), the density ($
ho$) decreases.

• Heat is a form of energy, not a physical substance that "takes up space" between molecules. So the first option is incorrect.
• The mass (and thus weight, since $W = mg$ where $g$ is constant near the Earth's surface) of an object does not change just because its temperature changes (unless there is a phase change that involves mass transfer, which is not the general case here). So the second option is incorrect.
• Molecules themselves do not "expand" in the sense of getting larger in size. It is the space between molecules that changes. So the third option is incorrect.

Answer:

Increased energy results in molecules moving farther apart.