Final answer:
A Category 3 Medical certificate is valid for 2 years for individuals over 40 and for 3 years for those under 40.
Step-by-step explanation:
The validity of a Category 3 Medical certificate for aviation purposes differs based on the age of the holder. For individuals who are over 40 years old, the certificate is valid for 2 years. On the other hand, for those who are under 40 years old, the validity period is 3 years. This is reflected in the options provided, with the correct answer being D) Over 40: 2 years, Under 40: 3 years.
It's important to keep in mind that these periods are determined by aviation authorities to ensure the safety and health standards of pilots and air traffic controllers are maintained.