What does internal energy (U) encompass?

Internal energy (U) is a thermodynamic property that represents the total energy contained within a system. It encompasses all the molecular-scale energies associated with the particles in a substance. This includes not only the kinetic energy of the particles, which relates to their motion, but also potential energy, which pertains to the interactions and positions of particles relative to one another.

In this context, all forms of energy at the molecular level contribute to internal energy: translational, rotational, vibrational kinetic energy, and potential energy from intermolecular forces. Therefore, the answer correctly identifies that internal energy encompasses the full range of molecular scale energies, making it the most comprehensive and accurate choice among the options presented.