The correct answer is A. Retaining wall.
A retaining wall is a structure that is built to resist the lateral pressure of soil or other materials. It is typically used to support a slope or embankment, or to retain water in a reservoir or canal. Retaining walls can be made of a variety of materials, including concrete, masonry, and steel.
A breast wall is a low retaining wall that is built to protect an embankment or slope from erosion. It is typically made of stone or concrete, and is usually less than 1 meter high.
A buttress is a structure that is built to support a wall or other structure. It is typically made of masonry or concrete, and is usually located at the base of the wall.
A parapet wall is a low wall that is built along the edge of a roof or other elevated structure. It is typically made of masonry or concrete, and is designed to protect people from falling.