Final answer:
A) annually
The frequency of medical validity checks for pilots depends on their age and the class of medical certificate they hold, with more rigorous requirements for older pilots and those holding higher-class certificates.
Step-by-step explanation:
The frequency of medical validity checks for a single pilot, regardless of their years of experience, is determined by age and the type of medical certificate they hold. For a pilot under the age of 40, a third-class medical certificate—which is what private pilots typically hold—is valid for 60 months (5 years). Once a pilot turns 40, the duration for the same certificate validity reduces to 24 months (2 years). For commercial pilots with a first-class medical certificate, checks are required annually if they are under 40, and every 6 months if they are 40 or older.