Final answer:
The best description of thermal energy is that it is the internal energy due to the motion of atoms and molecules, a form of kinetic energy. As molecules move faster, their kinetic energy and the substance's temperature increase. Hence, the correct answer is option (d).
Step-by-step explanation:
The statement that best describes thermal energy is: It is the internal energy of a substance due to the motion of its atoms and molecules. Thermal energy—often referred to as heat energy—is, indeed, the average kinetic energy of particles within a system. The faster these molecules move, the higher their kinetic energy, resulting in increased thermal energy and consequently a temperature rise. Therefore, when we refer to an object as 'hotter,' it typically means it possesses more thermal energy compared to a 'colder' object that has lesser kinetic energy in its particles.
It's important to note that thermal energy is only one component of internal energy, which also includes potential energy. Conversely, other forms of energy, such as nuclear energy, chemical energy, and electrical energy, are present in different contexts and under different conditions within substances.