The vertical part of the step that works as its support is:

Correct option is C

The riser is the vertical part of a stair step that provides the support between one tread and the next. It connects the horizontal parts (treads) and forms the vertical face of each step, giving stability and defining the height of each step.
Information Booster: In staircase construction, the two main components of a step are the tread and the riser. The tread is the horizontal part on which the foot is placed, while the riser is the vertical component supporting the tread above it. Risers can be open (without vertical boards) or closed, depending on the design. The riser contributes to the stair's strength and affects the overall rise and run of the staircase, influencing safety and comfort in climbing.
Additional Knowledge:
· (a) Newel: A structural post at the start or turn of a staircase, providing support for the handrail, not the vertical step part.
· (b) Tread: The horizontal surface of a step where the foot is placed, not vertical.
· (d) Soffit: The underside or horizontal surface beneath a stair or arch, not part of the step’s vertical support.