Which device has a movable core?

The device that has a movable core is the solenoid. A solenoid is essentially a coil of wire that creates a magnetic field when an electric current passes through it. Its defining feature is the movable core, usually made of ferromagnetic material, which can slide in and out of the coil. When electric current flows through the solenoid, the magnetic field generated pulls or pushes this core, which can be harnessed to perform mechanical work, such as actuating a switch or valve.

In contrast, relays function through an electromagnetic coil and a set of contacts that open or close, but they do not have a core that moves in the same manner as a solenoid. Transformers rely on static magnetic fields to transfer energy between coils of wire without a movable part. Additionally, capacitors store electrical energy in an electric field and also lack any moving components. Therefore, the solenoid stands out as the only device in the list that features a movable core, leading to its selection as the correct answer.