What does internal energy refer to?

Internal energy refers to the total energy associated with the microscopic components of a system, particularly the kinetic and potential energy resulting from the interactions among the particles – atoms and molecules – within that system. This includes both the energy in the form of kinetic energy due to the motion of these particles and the potential energy arising from the attractive and repulsive forces as the particles interact with one another.

Considering the other choices, while energy stored in chemical bonds is indeed a component of internal energy, it doesn't encompass all forms of energy present within a system. Gravitational forces, which are suggested in the first choice, specifically relate to external forces rather than the internal interactions of particles. On the other hand, energy due to the motion of an object is a separate concept, primarily concerned with macroscopic motion rather than the microscopic scale that internal energy describes. Thus, the comprehensive view of internal energy is best captured by the total energy from the attractive and repulsive forces among particles in a system.