Question

how does an increase in thermal energy affect molecules?
they stop moving.
they begin to vibrate more.
they decrease in temperature.
they lose kinetic energy.

Explanation:

Thermal energy is related to the motion of molecules. When thermal energy increases, according to the kinetic - molecular theory, the kinetic energy of molecules increases. Kinetic energy is associated with motion. Vibration is a form of molecular motion. An increase in thermal energy means more energy for molecular motion, so molecules vibrate more.
Stopping moving would mean a decrease in thermal energy (towards absolute zero). Decreasing in temperature is the opposite of increasing thermal energy. Losing kinetic energy is also contrary to the effect of increasing thermal energy.

Answer:

They begin to vibrate more.