The minimum width of a stair in residential buildings is 85 cm. This is according to the International Residential Code (IRC), which is a set of building codes that are used in the United States. The IRC requires that the clear width of a stair be at least 85 cm, measured from the face of the handrail to the face of the wall or other obstruction. This width is necessary to allow two people to pass each other comfortably on the stairs.
Option A, 55 cm, is too narrow for a stair in a residential building. This width would not allow two people to pass each other comfortably, and it would be difficult for people to carry items up or down the stairs.
Option B, 70 cm, is a common width for stairs in residential buildings. However, it is not the minimum width required by the IRC.
Option C, 85 cm, is the minimum width required by the IRC for stairs in residential buildings. This width is necessary to allow two people to pass each other comfortably on the stairs.
Option D, 100 cm, is a wider width that is sometimes used for stairs in residential buildings. This width provides more space for people to move around on the stairs, and it can be helpful for people with disabilities.