Final answer:
In a cyclic process, the internal energy of an ideal monoatomic gas remains constant because it returns to its original state, and internal energy is a function of temperature which is unchanged over the complete cycle.
Step-by-step explanation:
In a cyclic process, an ideal monoatomic gas goes through a series of changes that eventually return the gas to its initial state. As the process is cyclic, the answer to what happens to an ideal monoatomic gas is c) Its internal energy remains constant. This is because the internal energy of an ideal gas is solely a function of its temperature, and since the gas returns to its original state, the net change in temperature and, consequently, internal energy is zero over one complete cycle. However, during the cycle, the gas can absorb heat in certain processes and release heat in others. Isothermal processes, for example, are characterized by constant temperature; therefore, in such a process, any heat added to the system is used to do work on the surroundings, with no change in internal energy.