Which of the following defects would prevent the use of rear tires?

The presence of unrepaired fabric in a tire is a significant defect that would render the tire unsuitable for use. When the fabric, which provides structural integrity and strength to the tire, is damaged, it can lead to a loss of support and increase the risk of tire failure. This type of damage typically means that the tire cannot safely withstand the pressures of driving, especially under load, such as when carrying students.

In contrast, while baldness can significantly reduce traction, a puncture can often be repaired, and low air pressure can usually be remedied with inflation. Therefore, while those issues are serious and require attention, unrepaired fabric represents a failure of the tire's fundamental safety characteristics, warranting immediate replacement before the bus can operate safely.