What might indicate that a motorcycle's tires are under-inflated?

A motorcycle that is difficult to handle and keeps drifting is a strong indication that the tires may be under-inflated. When tires lack proper air pressure, it leads to an increase in the contact patch with the road, which can cause decreased stability and agility. This instability can manifest as handling difficulties and a tendency for the motorcycle to drift, especially during turns or while changing lanes. Properly inflated tires ensure optimal grip on the road, making the motorcycle easier to handle.

The other choices describe situations that do not necessarily correlate with under-inflated tires. Handling well and staying straight would more likely indicate that the tires are adequately inflated. Visibly flat tires are indeed a sign of under-inflation, but this may not always be as apparent when simply checking performance. Accelerating slowly could be due to various factors unrelated to tire pressure, such as engine performance or rider weight. Thus, the difficulty in handling and drifting is the most direct indicator of a potential issue with tire inflation.