How is the efficiency of instrument calculated?

The correct answer is option 4): \(\rm Instrument\ efficiency = \frac {Measuring \ quantity \ at \ full \ scale} {Power \ taken \ by \ the \ instrument \ at \ full \ scale}\) 

Concept:

The efficiency of instruments is the ratio of voltage and current rating to the power rating.

For an ammeter, the current rating is considered and for a voltmeter, the voltage rating is considered

\(\rm Instrument\ efficiency = \frac {Measuring \ quantity \ at \ full \ scale} {Power \ taken \ by \ the \ instrument \ at \ full \ scale}\)

Power taken by the is given by

For Ammeter: P = I²R

For Voltmeter: P = \(V^2 \over R\)