The correct answer is: C. Mullion
A mullion is a vertical structural member that divides a window or door opening into smaller panes or lights. It is typically made of wood, metal, or stone, and is often decoratively carved or molded.
A style is a vertical member that supports the lintel of a door or window opening. It is typically made of wood or stone, and is often decorated with moldings or carvings.
A reveal is the exposed area between the frame of a door or window and the surrounding wall. It is typically painted or stained to match the wall color.
A post is a vertical structural member that supports a beam or lintel. It is typically made of wood, metal, or stone, and is often square or rectangular in shape.