When should tires be replaced according to the measurement in two adjacent main grooves?

The correct choice indicates that tires should be replaced when the tread depth in two adjacent main grooves measures 5/32 of an inch. This measurement is significant because it is a recognized threshold for safe tire performance. Tread depth is crucial for maintaining grip on the road, particularly in wet conditions where hydroplaning can occur. At 5/32", the tire may not provide adequate traction, potentially compromising safety.

The importance of monitoring tread depth lies in understanding that as tires wear down, their ability to channel water away from the contact patch diminishes, increasing the risk of losing traction. While tires can still function adequately at greater depths, falling below the 5/32" mark can increase the likelihood of reduced performance during braking and cornering, making it essential to replace them when this measurement is reached.