The correct answer is A. Purlin.
A purlin is a horizontal structural member that supports common rafters of a sloping roof. It is typically made of wood, steel, or concrete, and is installed parallel to the rafters. Purlins are used to distribute the weight of the roof and its contents evenly across the rafters, and to provide lateral support for the rafters.
A cleat is a small, flat piece of wood or metal that is used to fasten two other pieces of wood or metal together. It is typically installed perpendicular to the pieces it is fastening, and can be used for a variety of purposes, such as attaching a rafter to a wall or ceiling.
A batten is a narrow strip of wood or metal that is used to cover joints or gaps between two pieces of wood or metal. It is typically installed perpendicular to the pieces it is covering, and can be used for a variety of purposes, such as covering the joints between shingles on a roof.
A strut is a structural member that is used to support a load by compression. It is typically made of wood, steel, or concrete, and is installed perpendicular to the load it is supporting. Struts are used in a variety of structures, such as bridges, buildings, and machines.