The correct answer is A. Thermocouple.
A thermocouple is a device that converts temperature to an electrical voltage. It is made of two dissimilar metals that are joined together at one end. When the temperature at the junction changes, an electric current is generated. The current is proportional to the temperature difference between the two junctions.
A potentiometer is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If the voltage across the resistor is known, the voltage across the sliding contact can be used to measure the resistance. The resistance can be varied by moving the sliding contact, which changes the ratio of the two resistors in the divider.
A gear train is a system of gears that transmit power from one part of a machine to another. The gears are connected by shafts, and the teeth of the gears mesh together to transfer power. The gear ratio determines how much the speed and torque of the output shaft changes compared to the input shaft.
An LVDT (linear variable differential transformer) is a device that converts linear motion to an electrical signal. It consists of a primary coil, a secondary coil, and a magnetic core. The core is movable, and as it moves, it changes the inductance of the secondary coil. This change in inductance is detected by the primary coil, and it is converted into an electrical signal.
None of the above options directly converts temperature into voltage. A thermocouple is the only option that directly converts temperature into voltage.