The correct answer is C) avoid rapid temperature changes and follow recommended operating procedures. The key to assuring a long service life for boiler refractory materials is to avoid subjecting them to rapid temperature changes. Refractory materials are designed to withstand high temperatures, but sudden changes in temperature can cause cracking and degradation over time. Following the manufacturer's recommended operating procedures, which typically call for gradual heating and cooling, is the most effective method to preserve the integrity of the refractory and extend its service life. The other options are not as effective. A) patching with plastic chrome ore may provide temporary repair but does not address the underlying issue of thermal stress. B) Securing the refractory with anchor bolts is important, but does not prevent temperature-related damage. D) Maintaining a high furnace temperature at all times is not recommended, as refractory materials require a more controlled thermal environment.