When does the E-series router start monitoring DHCP traffic using the DHCP External function?

When does the E-series router start monitoring DHCP traffic using the DHCP External function?

A.
When the discovery packet enters the router from the DHCP client.

B.
Only when the offer packet enters the router from the DHCP server.

C.
When the acknowledge packet enters the router from the DHCP server.

D.
Only when the discovery packet leaves the router going to the DHCP server.