Which of the following components is necessary for proper functioning in a closed expansion tank system?

In a closed expansion tank system, a circulating pump is essential for ensuring proper functioning. The purpose of the circulating pump is to move the fluid throughout the system. It enables the transfer of heat by drawing water from the expansion tank, pushing it through the system, and returning it effectively. This circulation is crucial because it maintains the required temperature and pressure within the tank and the rest of the system, ensuring efficient thermal regulation and preventing stagnation.

The other components mentioned, such as the expansion valve, pressure switch, and condensing unit, may play important roles in other types of systems or in specific functions within a broader setup but are not the primary component that enables the closed expansion tank system to operate effectively. Within this context, the circulating pump is therefore the key component necessary for the system's operation.