Final answer:
A third-class medical certificate issued to a 51-year-old pilot on May 3 is valid until midnight on May 31, 2 years later.
Step-by-step explanation:
A third-class medical certificate issued to a 51-year-old pilot on May 3 will be valid until midnight on May 31, 2 years later. This means that the validity period of a third-class medical certificate for a 51-year-old pilot issued on May 3 is option 1) Until midnight on May 31, 2 years later.