Internal energy (option A) is a state function because it depends only on the current state of the system and not on the path taken to reach that state. It is defined as the sum of the kinetic and potential energies of all the particles in the system.

Molar enthalpy (option D) is also a state function because it depends only on the current state of the system. Enthalpy is defined as the internal energy plus the product of pressure and volume, and it represents the heat content of a system at constant pressure.

On the other hand, irreversible expansion work (option B) and reversible expansion work (option C) are not state functions.

Irreversible expansion work is the work done on or by a system during an irreversible process, where the system is not in equilibrium with its surroundings. The amount of work done depends on the path taken during the process and is not solely determined by the initial and final states of the system.

Reversible expansion work, on the other hand, is the work done on or by a system during a reversible process, where the system is in equilibrium with its surroundings at every step. Similar to irreversible expansion work, the amount of work done depends on the path taken during the process and is not solely determined by the initial and final states of the system.

In summary, state functions are properties of a system that depend only on its current state and not on the path taken to reach that state. Internal energy and molar enthalpy are examples of state functions, while irreversible expansion work and reversible expansion work are not state functions.