What is an indicator of improperly adjusted air brakes?

Answer: B — Increased stopping distance

Increased stopping distance is a key indicator of improperly adjusted air brakes. When the air brake system is not correctly calibrated or adjusted, it can lead to a delayed response when the driver applies the brakes. This results in a longer distance required to stop the vehicle safely. If the brakes aren't engaging properly due to issues like improper slack adjustment or malfunctioning components, it can directly affect the effectiveness of the braking system. In contrast, other factors such as reduced fuel efficiency, frequent air leaks, and abnormal noise during braking may signal different issues within a vehicle's braking system or other mechanical aspects but do not specifically indicate an adjustment problem in the air brakes themselves. Understanding the relationship between brake adjustment and stopping distance is crucial for maintaining vehicle safety and performance.