Final answer:
Open risers in stair design must not allow a sphere larger than 4 inches in diameter to pass through to adhere to safety codes and prevent accidents.
Step-by-step explanation:
Open risers should not allow the passage of a sphere that is 4 inches in diameter. This requirement is part of building safety codes to prevent the risk of small children getting stuck or falling through the gap in the stairs. It is an essential aspect of stair design and construction that ensures a safe environment, especially in public and residential buildings. Gaps larger than 4 inches can pose a significant safety hazard.