Rectifier instruments are the type of instruments that are used to measure high-frequency AC signals. They are capable of measuring frequencies up to several megahertz (MHz). Among the given options, rectifier instruments have the highest frequency range with accuracy within reasonable limits.

Rectifier instruments work based on the principle of rectification, where the AC signal is converted into a DC signal. This conversion allows rectifier instruments to measure high-frequency AC signals accurately.

Let's discuss the other options and why they are not the correct answer:

a) PMMC (Permanent Magnet Moving Coil) instruments: PMMC instruments are used to measure DC (Direct Current) and low-frequency AC (Alternating Current) signals. They are not suitable for high-frequency measurements.

b) Moving iron instruments: Moving iron instruments are also used to measure DC and low-frequency AC signals. They have a limited frequency range and are not suitable for high-frequency measurements.

c) Electrodynamometer instruments: Electrodynamometer instruments are used for measuring both AC and DC signals. However, their frequency range is limited compared to rectifier instruments. They are not capable of accurate measurements at high frequencies.

Therefore, the correct answer is option 'D' - Rectifier instruments. They have the highest frequency range among the given options and are capable of accurate measurements within reasonable limits.

In conclusion, if we require an instrument that can measure high-frequency AC signals with accuracy, the rectifier instrument is the most suitable option. It can accurately measure frequencies up to several megahertz, making it ideal for high-frequency measurements.