C and D are correct

Correct answers are C & D. BFD tuning for BGP is described on page 204 in the Study Guide. - Enable BFD for faste failure detection (in less than 1 second) - For BFD multihop path, configure neighbor with; set ebgp-enforce-multihop enable, this is optional and needed IF you need support for multiple hops to reach your peer. As Artbrut mentions further down, it is stated pretty clearly in the link they have provided that peers must be on the same network for BFD to properly function, HOWEVER, this is for the OSPF configuration. The source in the link they provide is this; https://docs.fortinet.com/document/fortigate/7.2.0/new-features/729892/bfd-for-multihop-path-for-bgp Here it is stated that BFD only supported directly connected neighbors previously, but come 7.2.x it now supports multihop. Thus D is the correct second answer for this question.

Answer CD : p204 "It is independent of the type of media and dtects a one-way device failure in less than a second". "BFD was initially supported for twor routers directly connected on the same subnet. Now FortiGate can also support neighbors with BFD connected over multiple hops."

C,D https://docs.fortinet.com/document/fortigate/7.2.3/administration-guide/771813/bfd BFD for Multihop paths FortiGate BFD can support neighbors connected over multiple hops. When BFD is down, BGP sessions will be reset and will try to re-establish neighbor connection immediately. See BFD for multihop path for BGP for more information.

study guide page 204

have to revert, as per 7.2 multiple-hop bfd is supported! So c + d

https://community.fortinet.com/t5/FortiGate/Technical-Tip-FortiGate-BFD-implementation-and-examples/ta-p/190484 A -> nope, detects one-way device failure B -> yes as per link C -> yes as per link D -> RFC5883 describes multiple-hop-bfd, but I think in context of this question it does not apply as it is under circumstances