On heating an object, the speed of its molecules will increase. Heating transfers thermal energy to the molecules, causing them to move more rapidly; option [A]. This increase in molecular motion results in a rise in temperature and expansion of the object. Therefore, the correct option is [A] will increase, aligning with the principles of kinetic theory, which state that the average kinetic energy of molecules in a substance is directly proportional to its temperature. As the molecules gain energy, they move faster, leading to an increase in speed. This phenomenon is observed in various contexts, from the expansion of gases to the melting of solids and the vaporization of liquids. Thus, heating induces greater molecular motion, demonstrating the relationship between thermal energy and the speed of molecules in a substance.