The correct answer is D. Tread.
A tread is the horizontal surface of a step in a staircase. It is the part of the step that you walk on. The tread is typically wider than the riser, which is the vertical surface of the step. The rise and run of a step are the two dimensions that are used to measure the size of a step. The rise is the vertical distance from the top of one tread to the top of the next tread. The run is the horizontal distance from the front edge of one tread to the front edge of the next tread.
The tread is an important part of a staircase because it is what you walk on. The tread should be wide enough to accommodate your foot and should be level so that you do not trip. The tread should also be made of a material that is slip-resistant.
The other options are:
- Rise: The rise is the vertical distance from the top of one tread to the top of the next tread. The rise is typically measured in inches or centimeters.
- Flight: A flight of stairs is a series of steps that lead from one level to another. A flight of stairs typically consists of one or more risers and treads.
- Winder: A winder is a step that is not rectangular in shape. Winders are often used in spiral staircases.