As a minimum, how often must GTETS Operators and Qualifiers run any type/model aircraft engine?

The correct answer to how often GTETS Operators and Qualifiers must run any type/model aircraft engine is 90 days. This requirement ensures that personnel maintain their proficiency and the engines remain in proper operational condition. Regular operation of aircraft engines not only helps to keep the operators' skills sharp but also allows for the identification of any potential issues in a timely manner. This frequency is critical for maintaining the safety and reliability of flight operations. The interval of 90 days balances the need for practice with operational and maintenance schedules.

Other options, such as 30 days, 6 months, or 12 months, would not ensure the same level of readiness and proficiency in operating and qualifying personnel on the engines, resulting in potential skill degradation or unnoticed issues in engine performance.