The correct answer is A. Cleat.
A cleat is a small piece of wood or metal that is fixed to a surface to provide support or to prevent movement. In the case of a door frame, a cleat is often used to prevent the door from swinging too far open.
A stop is a piece of wood or metal that is fixed to a surface to prevent something from moving past a certain point. In the case of a door frame, a stop is often used to prevent the door from swinging too far closed.
A horn is a projection on a piece of furniture or other object that is used for decoration or to provide a handle. In the case of a door frame, there are often horns on the top and bottom of the frame that are used to hold the door in place.
None of these is the correct answer because it does not describe a wooden block fixed on the back side of a door frame on its post.