Which of the following best defines the internal energy of a system?

a.
the internal energy is the difference of the kinetic energy due to the rotational, vibrational and translational motion of molecules and potential energy associated with the vibrational energy of atoms within molecules.

b.
the internal energy is the product of the kinetic energy due to the rotational, vibrational and translational motion of molecules and potential energy associated with the vibrational energy of atoms within molecules.

c.
the internal energy is the quotient of the kinetic energy due to the rotational, vibrational and translational motion of molecules and potential energy associated with the vibrational energy of atoms within molecules.

d.
the internal energy is the total of the kinetic energy due to the rotational, vibrational and translational motion of molecules and potential energy associated with the vibrational energy of atoms within molecules