Which of the following is a common cause of tire fires?

Underinflated tires are indeed a common cause of tire fires. When tires are underinflated, they can generate excessive heat as they flex more than they are designed to. This increase in heat can lead to a breakdown of the tire materials, ultimately resulting in a tire failure or fire. Since underinflated tires have a larger surface area that comes into contact with the road, they also create increased friction. This added heat from both the road contact and the flexing of the tire itself can reach a critical point, causing a fire.

Other options, such as overinflated tires, normal wear and tear, and routine maintenance, while they might present their own issues (like reduced traction or increased blowout risk), do not specifically contribute to tire fires as directly as underinflation does. Overinflated tires can lead to blowouts due to increased pressure but do not typically create the same heat buildup associated with underinflation. Normal wear and tear and routine maintenance, when conducted properly, help to ensure tire safety and do not inherently lead to tire fires.