The correct answer is: A. electrostatic instrument.
An electrostatic instrument cannot be designed on the principle of electrostatics because it cannot measure power. Power is the rate at which energy is transferred, and electrostatic instruments can only measure voltage or current.
A thermocouple instrument can be designed on the principle of thermoelectricity, which is the conversion of heat into electricity. A thermocouple instrument consists of two dissimilar metals that are joined together at one end. When the junction is heated, an electric current is generated. The current is proportional to the temperature difference between the two junctions.
A moving iron instrument can be designed on the principle of magnetism. A moving iron instrument consists of a coil of wire that is placed in a magnetic field. When current flows through the coil, it creates a magnetic field that interacts with the external magnetic field. This interaction causes the coil to move. The amount of movement is proportional to the current flowing through the coil.
An electrodynamic instrument can be designed on the principle of electromagnetism. An electrodynamic instrument consists of two coils of wire that are placed close together. When current flows through one coil, it creates a magnetic field. This magnetic field interacts with the current flowing through the other coil, causing the two coils to move. The amount of movement is proportional to the product of the currents flowing through the two coils.