Which of the following devices is primarily used for protecting electrical circuits?

The fuse is the primary device used for protecting electrical circuits. It serves the critical function of safeguarding circuits from overcurrent situations, which can lead to overheating, damage, and in worst cases, fire. When an electrical current exceeds the designated level, the fusible link within the fuse melts, breaking the circuit and stopping the flow of current. This prevents further damage to the circuit components and maintains a level of safety in the electrical system.

In contrast, the other devices listed have different roles. A capacitor is used to store and release electrical energy, typically affecting voltage levels and stabilizing electrical systems, but it does not provide protective functions. An inductor, on the other hand, is primarily used for storing energy in a magnetic field, often utilized in filters and oscillators, but it does not act as a safety device. Lastly, a transformer is designed to change voltage levels in AC circuits, allowing for efficient electricity transmission and distribution, but again, it does not serve a protective role for the circuit itself. Thus, the unique protective function of the fuse makes it the correct answer in this context.