When an object is cooled, the energy of its molecules decreases; option [B]. This reduction in energy means that the thermal energy, which is associated with the random motion of the molecules, is lowered. As a result, the molecules move more slowly. Hence, the speed of the molecules decreases. Cooling an object reduces the kinetic energy of its molecules, directly impacting their speed and leading to a slower movement. The mass and weight of the object remain unchanged during this process, as cooling primarily affects the thermal energy and motion of the molecules rather than their mass or weight. Therefore, the correct option is [B] speed decreases. This concept is fundamental in understanding how temperature changes influence the behavior of molecules in various states of matter.