For high-power AC measurements, instrument transformers are integral when using a wattmeter due to their specific advantages. Let's explore the reasons step-by-step:

1. Purpose of Instrument Transformers:
   • Instrument transformers, such as Current Transformers (CTs) and Potential Transformers (PTs), are employed to scale down the high currents and voltages in power circuits to lower values that are safe to measure and handle with standard instruments like a wattmeter.
2. Safety and Range Extension:
   • These transformers extend the measurement range of the wattmeter. By stepping down high currents and voltages, they prevent potential damage to the wattmeter, ensuring safe and accurate measurements.
3. Benefits over Direct Measurement:
   • Directly measuring high power levels without transformers would require wattmeters of impractical size and cost due to the extreme current and voltage ranges in industrial settings.
4. Other Options Explained:
   • To eliminate the need for a pressure coil in the wattmeter: This is incorrect; a pressure coil is necessary for the measurement operations of a wattmeter.
   • To reduce the frequency of the measured circuit: Frequency reduction is not a role of instrument transformers.
   • To convert AC power into DC power: Instrument transformers do not perform AC to DC conversion, which is the role of rectifiers.

Given these explanations, the correct reason for using instrument transformers with a wattmeter in high-power AC measurements is to extend the current and voltage range of the wattmeter safely.