The correct answer is: A. moving coil instrument.
A Weston frequency meter is a type of moving coil instrument that is used to measure frequency. It consists of a coil of wire that is suspended in a magnetic field. When an alternating current is passed through the coil, it produces a force that causes the coil to rotate. The frequency of the current is proportional to the speed of rotation of the coil.
A moving iron instrument is a type of instrument that uses a moving iron to measure an electrical quantity. The iron is attracted to or repelled by a magnetic field, and the amount of movement is proportional to the quantity being measured. Moving iron instruments are typically used to measure current, voltage, and resistance.
A dynamometer instrument is a type of instrument that uses a dynamometer to measure an electrical quantity. A dynamometer is a device that measures the force between two magnetic fields. The force is proportional to the square of the current flowing through the coils of the dynamometer. Dynamometer instruments are typically used to measure power and energy.
The answer to the question is A. moving coil instrument because a Weston frequency meter is a type of moving coil instrument.