The correct answer is C because the requirement is to monitor the traffic sent to the server. In the diagram, the server is connected downstream through the intermediate switch, and N9K1 connects toward that path on interface Eth1/1. On Cisco NX-OS SPAN, the direction keyword is important: tx captures frames transmitted out of the selected source interface, while rx captures frames received into that interface.

Since traffic destined for the server leaves N9K1 out Eth1/1, the proper source is source interface eth1/1 tx. That mirrored traffic is then forwarded to the analyzer connected on the SPAN destination interface.

Option D is incorrect because rx on Eth1/1 would monitor traffic arriving from the downstream switch toward N9K1, not traffic being sent to the server. Option A is incorrect because Eth1/3 is on the downstream switch, not on N9K1. Option B uses an invalid interface reference for this topology.

Therefore, to capture traffic heading from N9K1 toward the server, the correct SPAN source on N9K1 is Eth1/1 in the transmit direction.