Concept:An ammeter is the standard device for measuring current, but among the given options, a CRO is the only instrument that can measure current.
Explanation:A Cathode Ray Oscilloscope (CRO) is a versatile instrument used to measure voltage, current, frequency, and phase.
To measure current with a CRO, the current is passed through a known resistor and the voltage drop is measured using the relation
V=IR.
The Wattmeter measures electric power, the Ohm meter measures resistance, and the Energy meter measures the total energy consumed.
Hence, out of the given choices, the CRO is the correct instrument for measuring current.
Answer:D. CRO