The correct answer is B. Meggar.
A meggar is an instrument used to measure the insulation resistance of an electric circuit relative to earth and one another. It is a high-voltage, low-current device that applies a test voltage to the insulation of the circuit and measures the current that flows. The insulation resistance is then calculated by dividing the test voltage by the current flow.
A tangent galvanometer is a device used to measure the tangent of the angle between a magnetic field and a current-carrying conductor. It is not used to measure insulation resistance.
A current transformer is a device used to measure the current in a circuit without having to connect directly to the circuit. It does this by using a magnetic field to induce a voltage in a secondary coil. The voltage in the secondary coil is proportional to the current in the primary coil. Current transformers are not used to measure insulation resistance.
None of the above is the correct answer.