Which of the following is a critical factor in the maintenance of air brakes?

Answer: B — Frequent inspection of the air compressor.

The frequent inspection of the air compressor is essential for the effective maintenance of air brakes because the compressor is responsible for generating the compressed air needed to operate the braking system. If the air compressor is not functioning properly, it can lead to insufficient air pressure in the system, which directly affects braking performance. Regular checks ensure that the compressor is working efficiently, preventing issues such as air leaks, inadequate pressure, or complete compressor failure. Maintaining the integrity and functionality of the air compressor is crucial for ensuring safety and reliability in vehicles equipped with air brakes. Other factors, like tire pressure or brake fluid levels, while important, do not directly pertain to the air brake system itself, making them less critical for air brake maintenance.