Final answer:
The guards on the open side of stairs must not have openings that allow the passage of a sphere 4 inches in diameter to ensure child safety.
Step-by-step explanation:
The question pertains to the safety regulations concerning guards on stairs, which is a topic typically covered in engineering and architecture studies, specifically within the context of building codes and construction standards. Guards on the open side of stairs are designed to prevent falls, and according to the International Building Code (IBC), as well as various local building codes, they shall not permit the passage of a sphere that is 4 inches (102 mm) in diameter or larger.
This requirement serves to prevent small children from slipping through the openings and ensures a certain level of safety for all users of the stairs.