Final answer:
The correct option is C) can be adjusted manually or automatically. Slack adjusters in vehicle braking systems can be adjusted either manually or automatically to maintain proper brake function, and should be adjusted.
Step-by-step explanation:
The correct answer is C) can be adjusted manually or automatically. Slack adjusters are an essential component in the braking systems of heavy vehicles like trucks and buses. They ensure that the brakes are not too loose or too tight by adjusting the distance between the brake shoes and the drum. There are two types of slack adjusters: manual and automatic. Manual slack adjusters require periodic adjustments by a mechanic to maintain proper brake function. On the other hand, automatic slack adjusters adjust themselves during normal brake application, but may still require occasional manual adjustments or inspections to ensure they are functioning correctly.
It is not accurate to say that slack adjusters never need adjustment (A) or that their travel should be at least two inches (D); this would indicate a malfunctioning adjuster or potential brake failure. Additionally, while a qualified mechanic should perform adjustments to ensure safety, some regulations may allow for drivers trained in proper adjustment techniques to adjust manual slack adjusters as needed.