What part of the stair supports the tread?

The stringer is the structural component of a staircase that supports the treads and risers, effectively forming the backbone of the staircase. It is typically a long, inclined board that is cut to accommodate the placement of treads and risers. As the treads are the horizontal parts of the staircase where people step, the stringer plays a crucial role in providing the necessary support and stability for these treads.

The riser, while it does contribute to the overall structure of the staircase, is the vertical component that sits between each tread and does not directly support the tread itself. The handrail is intended for user support and safety, providing a graspable element along the staircase, but it does not contribute to the structural support of the treads. The landing is a flat area at the top or bottom of a staircase, providing a transition space but not directly involved in the support of the treads. Thus, the stringer serves as the essential support for the treads, confirming the chosen answer.