Electromechanical devices
Electromechanical devices are devices that convert electrical energy into mechanical energy or vice versa. These devices typically involve the interaction of electrical and mechanical components to perform a specific function. Some common examples of electromechanical devices include motors, generators, relays, and sensors.

Options
a) Induction relay: An induction relay is an electromechanical device that uses the principle of electromagnetic induction to control the flow of current in a circuit. It consists of a coil, a movable armature, and contacts. When the coil is energized, the magnetic field generated induces a current in the armature, causing it to move and open or close the contacts. Induction relays are commonly used for protective relaying in power systems.

b) Thermocouple: A thermocouple is a temperature sensor that generates a voltage proportional to the temperature difference between two junctions. It consists of two dissimilar metal wires connected at one end. When there is a temperature difference between the two junctions, a voltage is produced due to the Seebeck effect. Thermocouples are widely used for temperature measurement in various applications.

c) LVDT: An LVDT (Linear Variable Differential Transformer) is an electromechanical transducer that converts linear displacement into an electrical signal. It consists of a primary coil, two secondary coils, and a movable core. When the core moves along its axis, the voltages induced in the secondary coils change proportionally. LVDTs are commonly used for position sensing in applications such as robotics, aerospace, and industrial automation.

d) All of the mentioned: This option is incorrect because not all of the mentioned devices are electromechanical devices. Only option c) LVDT is an electromechanical device.

Conclusion
The correct answer is option c) LVDT. An LVDT is an electromechanical device that converts linear displacement into an electrical signal. Induction relays and thermocouples are also commonly used in electrical engineering, but they do not fall under the category of electromechanical devices.