What type of fitting is installed in a plumbing system to prevent the passage of air, gas, and some vermin through the pipe?

The type of fitting that is specifically designed to prevent the passage of air, gas, and some vermin through the pipe is indeed a trap. Traps are essential components in plumbing systems, primarily found in drains, and they create a water seal that blocks gases from seeping back into the living spaces. This water barrier is vital for maintaining sanitary conditions within a plumbing system by preventing foul odors and harmful gases, such as sewer gas, from entering homes or buildings.

In contrast, check valves are primarily used to allow fluid to flow in one direction only and prevent backflow, but they do not provide the water seal characteristic of traps. Flap valves serve a similar purpose to check valves and are designed to prevent backflow but again do not function to keep gases out from the system like traps do. Gate valves control the flow of water and can be used to isolate sections of piping but also do not create a barrier against air or gas infiltration. Thus, the unique function of traps in plumbing systems makes them the correct answer for this question.