Which of the following defects is classified as 'major'?

The classification of defects in vehicle inspection is crucial for ensuring safety on the road. In this case, the correct answer is the presence of exposed cords in the tread or sidewall of a tire. This is considered a major defect because exposed cords indicate that the tire has lost its structural integrity, significantly increasing the risk of a blowout or tire failure while driving. The lack of protective material means the tire is no longer safe for use, as it can't adequately handle the stresses and strains of driving.

In contrast, minor sidewall cracks may suggest some wear but do not immediately compromise the tire's safety. A tire that has proper tread depth is fundamentally sound and safe, fulfilling the required standards for traction. Similarly, while a leaking air valve stem is problematic and requires attention, it does not render the tire itself unsafe to the same extent as exposed cords would. Each of these factors plays a role in evaluation, but exposed cords distinctly indicate a critical safety concern necessitating immediate action.