Which of the following is the least likely cause of one brake dragging?

The least likely cause of one brake dragging is a corroded master cylinder bore. The master cylinder is responsible for generating hydraulic pressure to operate the brake system. While corrosion can affect the performance of the master cylinder and potentially lead to brake issues, it is more likely to cause problems with the overall braking system rather than causing one specific brake component to drag.

When the master cylinder is compromised, it typically results in issues like reduced braking efficiency or uneven braking across all wheels rather than isolating the issue to just one brake dragging. On the other hand, a faulty brake line, worn brake pads, or malfunctioning calipers are all more directly linked to causing one brake to drag. A faulty brake line could create uneven pressure, worn brake pads could lead to uneven wear and function, and malfunctioning calipers can cause them to stick, all of which would more likely lead to an individual wheel dragging rather than affecting the system as a whole.