Which statement correctly describes the free area requirements for ducts introducing combustion air?

The statement that indicates a requirement for a minimum of 100 square inches of free area for ducts introducing combustion air is accurate. This standard is based on the need to ensure adequate airflow for combustion processes, which is crucial for maintaining efficient operation and safety in heating systems.

Adequate combustion air supply is essential to prevent incomplete combustion, which can lead to the production of carbon monoxide and other hazardous byproducts. By mandating a minimum free area, the regulation helps to ensure there's enough air available for the burner, facilitating optimal combustion and reducing risks associated with low air supply.

While other options mention different criteria regarding the size and number of openings, they do not align with the established standards for ensuring sufficient combustion air. For example, specifying openings of any size or suggesting a specific ratio of square inches per BTU does not meet the straightforward requirement that ensures a baseline airflow is always available for combustion needs.