Correct answer: Configure an intracard or intercard ODUk cross-connection between the source and sink timeslots corresponding to the two client ports, and then enable a client-side inward loopback (terminal loopback) on the output port.
This is the correct and comprehensive procedure. First, an ODUk cross-connection is created to route the signal from the input port's timeslot to the output port's timeslot, which explicitly tests the switching fabric. Second, an inward (terminal) loopback is activated on the output port. This directs the signal that has traversed the fabric back into the tributary board's receiver. By sending a signal into the input port (from a test set or just by monitoring for errors on an active client signal), the engineer can verify the entire path shown in the diagram, confirming the functionality of both tributary cards and the cross-connect fabric.