Final answer:
The component that becomes thinner as the clutch wears is the friction disc. It wears down because of the constant friction and heat during the clutch operation.
Step-by-step explanation:
As the clutch wears, the component that becomes thinner is the friction disc. The friction disc, also known as the clutch disc, is positioned between the flywheel and the pressure plate. It has friction material on both sides that wears down over time due to the constant contact and heat generated when the clutch engages and disengages. The release bearing is a separate component that does not wear in the same manner as the friction material. Therefore, consistent use of the clutch leads to the thinning of the friction disc, which may eventually require replacement to maintain the performance and functionality of the vehicle's clutch system.