The correct answer is A. Reflux valve.
A reflux valve is a type of valve that allows water to flow in one direction but prevents its flow in the reverse direction. It is used to prevent backflow of water into a system, which can cause contamination or damage. Reflux valves are typically installed in plumbing systems, water heaters, and other applications where backflow prevention is necessary.
A sluice valve is a type of valve that is used to control the flow of water. It is typically used in irrigation systems and other applications where it is necessary to regulate the flow of water. Sluice valves are typically made of metal or plastic and have a gate that can be opened or closed to control the flow of water.
An air relief valve is a type of valve that is used to release air from a system. It is typically used in water heaters and other applications where it is necessary to prevent air from building up in a system. Air relief valves are typically made of metal or plastic and have a diaphragm that opens to release air from the system.
A pressure relief valve is a type of valve that is used to release pressure from a system. It is typically used in boilers and other applications where it is necessary to prevent pressure from building up in a system. Pressure relief valves are typically made of metal or plastic and have a diaphragm that opens to release pressure from the system.