Final answer:
In the context of directional couplers, the correct adjustment in the return paths when using one is to increase the directivity, which ensures the effective separation of forward and reverse power directions and improves coupler performance.
Step-by-step explanation:
The student's question is about the adjustments necessary in the return paths of a system when employing a directional coupler. A directional coupler is a four-port network where one port is isolated from the input port and is used to couple a certain amount of the power that is flowing from one line to another. The use of a directional coupler typically requires an increase in directivity. Directivity is a measure of how well the coupler can separate forward and reverse power directions. High directivity indicates that the coupler is very effective at preventing energy that is meant to go to the output from leaking into the isolated port. An increase in directivity usually comes with better performance of the directional coupler in terms of accuracy for measurement or signal separation purposes.