Which of the following is a state function?

A state function is a property of a system that depends only on its current state or condition, regardless of how that state was achieved. Internal energy is a key example of a state function because it is determined solely by the system's state variables, such as temperature, pressure, and volume. It does not depend on the specific process or path taken to change the system from one state to another.

In contrast, heat transfer and work done are not state functions; they depend on the specific process or path taken. For instance, the amount of heat transferred or work done can vary significantly depending on the method of the process, such as whether the reaction occurs at constant pressure or constant volume.

Time, while often important in thermodynamics and kinetics, is not considered a state function in the same context as internal energy. It does not describe the state of the system but rather measures the duration of processes.

Thus, internal energy stands out as the correct choice, as it distinctly satisfies the criteria of being a state function, independent of the way in which the system was altered.