The correct answer is D. ohmmeters and resistance bridges.
An ohmmeter is a device used to measure electrical resistance. It is a simple instrument that consists of a battery, a galvanometer, and a series resistor. The galvanometer is a device that measures current, and the series resistor is used to create a known voltage drop. When the ohmmeter is connected to a circuit, the current through the galvanometer is proportional to the resistance of the circuit. The galvanometer’s deflection is then calibrated to read resistance in ohms.
A resistance bridge is a more complex instrument that is used to measure the resistance of an unknown resistor. It consists of four resistors, a galvanometer, and a battery. The resistors are arranged in a bridge circuit, and the battery is connected to the bridge. When the bridge is balanced, the current through the galvanometer is zero. The resistance of the unknown resistor can then be calculated from the known values of the other resistors in the bridge.
Wattmeters, voltmeters, and ammeters are not used to measure resistance. A wattmeter measures power, a voltmeter measures voltage, and an ammeter measures current.