Final answer:
The most likely candidate to use as the basis for a primary key in the Airport table is airport code.
Step-by-step explanation:
The most likely candidate to use as the basis for a primary key in the Airport table would be airport code. This is because an airport code is a unique identifier assigned to each airport and it can uniquely identify each record in the Airport table. Other fields like address, city, and state may have duplicate values across different airports, making them less suitable for a primary key.