The correct answer is: A. Air relief valve.
An air relief valve is a type of valve that is used to allow air to escape from a system while preventing the flow of liquid. It is typically installed on the suction pipe of a tube-well to prevent air from entering the system and causing problems.
A reflux valve is a type of valve that is used to prevent the flow of liquid in the opposite direction. It is typically installed on the discharge pipe of a tube-well to prevent water from flowing back into the well.
A pressure relief valve is a type of valve that is used to prevent the pressure in a system from exceeding a certain level. It is typically installed on the discharge pipe of a tube-well to prevent the well from being damaged by excessive pressure.
A sluice valve is a type of valve that is used to control the flow of water. It is typically installed on the discharge pipe of a tube-well to control the amount of water that is being pumped out of the well.