Final answer:
A baffle is specifically mandatory for using a 3) diffusion pump, which functions to prevent vapor contamination in high vacuum systems.
Step-by-step explanation:
A baffle is only required when using a diffusion pump. The primary function of the baffle in such systems is to prevent the oil or other working fluid vapor from entering the vacuum chamber. Diffusion pumps are used to achieve high vacuums by allowing gas molecules to be captured by a vapor jet, but this can cause contamination if the vapor from the pump fluid reaches the chamber. Hence, baffles are used to condense and trap these vapors before they can cause any contamination. These types of pumps are particularly effective for creating ultra-high vacuum conditions but require a baffle system to ensure the purity and stability of the vacuum environment.