Which multiple line appearance parameter sets a number of calls per line beyond which all incoming calls are forwarded?

A.
Maximum Number of Calls

B.
Call Forward Busy

C.
No Answer Ring Duration

D.
Busy Trigger

Explanation:
Call Forward Busy Trigger
Some users might wish to have incoming calls routed to a CFB destination earlier than reaching the maximum number of calls allowed per line appearance. The Busy Trigger feature is present in Cisco CallManager 4.0 to satisfy these users’ requirements. If all the devices that share a particular line are busy, the call to that specific DN is rejected with a busy cause. If the CFB destination is set for that DN, this rejected call is routed to the CFB destination.
The Busy Trigger is database configurable per line appearance and per cluster. It cannot exceed the maximum number of calls set for this DN. The default value for MCD devices is set to 2. By default, this busy trigger is one for non-MCD devices. As the maximum number of calls is already limited to 2, this cannot be set for more than 2.
See
Table-2for a comparison of CFB in Cisco CallManager 3.x and Cisco CallManager 4.0.
Table-2

This Busy Trigger replaces the Call Waiting Flag per DN. During migration, if the Call Waiting Flag is set to true, the busy trigger is set to 2. Otherwise, it is set to 1.
Reference:
http://www.cisco.com/en/US/partner/products/sw/voicesw/ps556/products_tech_note09186a00801f3b4e.shtml# c
Note: B could also be correct