Which tool would be most appropriate to measure the thickness of a brake rotor?

The caliper is the most appropriate tool for measuring the thickness of a brake rotor because it is specifically designed to provide precise measurements of small dimensions. A caliper, particularly a digital or dial caliper, can easily measure the thickness of the rotor at various points to ensure it meets the manufacturer's specifications. Proper thickness measurement is crucial, as a rotor that is too thin can lead to overheating and brake failure, while one that is too thick may not fit properly within the brake system.

Using a torque wrench serves a different purpose, primarily for measuring the tightness of fasteners and bolts, which is not relevant to measuring rotor thickness. A feeler gauge is generally used to measure gaps or clearances and is not suitable for directly measuring the thickness of a solid object like a brake rotor. A pneumatic drill is a power tool used for drilling and has no application in measuring dimensions. Therefore, the caliper is the clear choice for accurately assessing rotor thickness.