What is required for a boiler to be classified as a high-pressure boiler?

A boiler is classified as a high-pressure boiler when it exceeds a pressure of 15 pounds per square inch gauge (psig). This classification is important because it involves different safety and operational regulations compared to low-pressure boilers. Specifically, any boiler that operates at a pressure greater than 15 psig is subject to stringent safety standards, inspections, and operational procedures to ensure safe and efficient operation.

The other criteria mentioned in the options do not solely suffice for the classification. While the temperature may be relevant, the primary defining characteristic for high-pressure status is indeed the pressure threshold. Therefore, option A aligns with the regulatory definitions and safety requirements associated with high-pressure boilers, making it the correct answer.