The correct answer is: A. Check valve.
A check valve is a type of valve that is used to prevent the flow of water in a pipe in the reverse direction. It is typically installed at street corners and where the pipe lines intersect to prevent water from flowing back into the distribution system.
A sluice valve is a type of valve that is used to control the flow of water in a pipe. It is typically installed in irrigation systems and canals to control the amount of water that is flowing.
A safety valve is a type of valve that is used to prevent the pressure in a system from becoming too high. It is typically installed in boilers and other pressurized systems to prevent them from exploding.
A scour valve is a type of valve that is used to remove sediment from a pipe. It is typically installed in storm drains and other drainage systems to prevent them from becoming clogged.