ANS:D - tread.

The horizontal portion of a step in a staircase is known as the "tread." Explanation:

• The "tread" is the flat, horizontal surface of a step where the foot rests while ascending or descending the staircase.
• It provides the area on which a person places their foot while using the stairs.
• The vertical portion of a step is called the "riser."
• "Flight" refers to a continuous series of steps between two landings.
• "Winder" refers to steps that are narrower on one side than the other, allowing a staircase to change direction without the use of a landing.