Final answer:
The peak reading element is required to measure percentage of modulation on the Bird Thruline Wattmeter Model 4391M, as it can detect peak to peak variations in the modulation envelope.
Step-by-step explanation:
The Bird Thruline Wattmeter/RF Power Analyst, Model 4391M, requires a peak reading element to measure percentage of modulation (% MOD). Neither the forward nor the reflected plug-in elements are used for this purpose. If one were to measure the actual power or reflected power, one would indeed use the corresponding forward and reflected plug-in elements. However, for measuring modulation percentages, it specifically requires the capability to detect peak to peak variations, which are indicative of the modulation envelope in an amplitude modulated signal.