Final answer:
Excessive wear on the edges of the tires indicates under-inflation. This wear pattern occurs due to increased friction on the edges when tire pressure is too low. Sidewall wear or center tread wear usually suggests other problems.
Step-by-step explanation:
Under-inflated tires can be identified by specific wear patterns on the tires themselves. The correct answer for what indicates under-inflated tires is a) Excessive wear on the edges of the tires. This happens because when tires don't have enough air pressure, the outer edges make more contact with the road surface compared to the center. This increased friction causes the edges to wear out faster than the middle.
By contrast, over-inflated tires will often show excessive wear in the center of the tread because the higher pressure causes the middle part of the tire to bulge and come into greater contact with the road. As for excessive wear on the sidewalls, that's typically an indicator of other issues, like driving habits or structural problems with the tire, rather than an issue of inflation level.
The relationship between tire pressure and tire behavior can be understood by considering how a tire behaves when it is inflated. As shown in Figure 13.18, initially as air is pumped into a deflated tire, the volume increases with little change in pressure (a). Once the tire reaches a point of optimal inflation, the tire walls resist further expansion and pressure increases with additional air (b). Finally, when a tire is fully inflated, its pressure can further increase with temperature (c).