What indicates a problem with the brake linings?

The option indicating that brake linings are worn dangerously thin is correct because excessively worn brake linings can significantly compromise a vehicle's braking performance, leading to a dangerous situation while driving. Adequate brake linings are essential for effective braking, as they create the friction necessary to stop the vehicle safely. When the linings are too thin, there may not be enough material left to generate the required friction, resulting in longer stopping distances and an increased risk of brake failure.

In contrast, visible lubricant on the linings may suggest a maintenance issue but does not necessarily indicate the effectiveness of the brake lining material itself. The fading color of the brake linings can indicate wear, but it does not provide a direct assessment of the actual thickness and safety of the linings. Similarly, dirt on the linings can be an indicator of various conditions but does not specifically relate to the critical measurement of how thin the brake linings have become or their performance capability. Thus, assessing the thickness of the brake linings is the most critical factor in determining their safety and functionality.