The Enterprise Firewall 7.6 Administrator Study Guide states: "You can also use the network command to configure FortiGate BGP to advertise prefixes." Therefore, FortiGate_C must originate the required prefix by adding 100.64.2.0/24 under config router bgp # config network.
By default, the configured prefix must exactly match an active route in FortiGate_C's routing table. The topology indicates that 100.64.2.0/24 is directly connected to FortiGate_C, so the network-import check succeeds and FortiGate_C advertises that specific prefix to FortiGate_B. FortiGate_B can then advertise the learned BGP route to FortiGate_A through the existing eBGP adjacency.
Connected-route redistribution on FortiGate_C would be too broad because it would redistribute all eligible connected routes, including the other connected subnet 100.65.3.0/24. A route-map-in on FortiGate_A can filter routes that FortiGate_A receives, but it cannot originate a route that FortiGate_C has not advertised.
FortiGate_B does not need to redistribute BGP into BGP; normal BGP route propagation handles the advertisement between autonomous systems.
Example configuration on FortiGate_C:
config router bgp
config network
edit 1
set prefix 100.64.2.0 255.255.255.0
next
end
end
References: Dynamic Routing Supplement - Network Command , page 346; Fortinet: BGP network command and exact-prefix requirement ; FortiOS 7.6.7 CLI Reference .

To identify the reason for the lack of prefixes, we must interpret the State/PfxRcd and Up/Down columns in the get router info bgp summary exhibit.
Analyze Neighbor Status:
Neighbor 10.125.0.60: State is OpenSent. This session is not established. It is stuck in the negotiation phase.
Neighbor 100.64.3.1: State is Active. This session is not established. The router is actively trying to initiate a TCP connection.
Neighbor 10.127.0.75:
Up/Down: 02:45:55. This indicates the BGP session has been Up (Established) for almost 3 hours.
State/PfxRcd: 0. This number represents the count of prefixes received. The session is fully established, but the neighbor has sent zero routes.
Determine the Cause:
Since the session with 10.127.0.75 is established, connectivity and handshakes (Options A, B, C) are not the issue for this neighbor.
The fact that it is Up but sending 0 prefixes strongly implies that the neighbor is configured to filter out its routes before sending them to the local FortiGate.
Option D correctly identifies this as a RIB-OUT (Routing Information Base - Outbound) configuration issue on the neighbor (Router 10.127.0.75), which prevents it from advertising its routes.
Reference:
FortiGate Security 7.6 Study Guide (BGP): " In the BGP summary, if the State/PfxRcd shows a number (e.g.,
0), the session is Established. A value of 0 means the peering is up, but no routes have been received, often due to route-map or prefix-list filtering on the remote peer. "

The session output includes the flags:
state=redir local may_dirty ...
npu_state=00000000
offload=0/0
The 7.6 study guide explains these flags directly:
local = "Session is to/from local stack"
redir = "Session is being processed by an application layer proxy"
may_dirty = "Session is allowed by a firewall policy"
This makes C correct, because the local flag means the session either originates from FortiGate or terminates on FortiGate. The FortiOS administration guide states the same meaning: "Session is originated from or destined for local stack." This also makes A correct. The redir flag means the session is handled by an application-layer proxy. FortiOS documents explain that proxy-based inspection buffers traffic on the FortiGate and inspects it there, and that proxy-based processing is CPU and memory-intensive Since the session also shows no NPU offload (npu_state=00000000, offload=0/0), this traffic is being handled in software/CPU, not by the NPU.
Why the other options are wrong:
B is wrong because the redir flag proves the session is not passing without inspection; it is being processed by an application-layer proxy D is wrong because there is no authentication flag in this session. In Fortinet examples of captive portal
/authentication-related sessions, the session state includes auth or authed flags. The study guide shows: "Any session for traffic coming from an authenticated user contains the authed flag." This exhibit does not show auth or authed, so there is no basis to conclude the client was redirected to a captive portal for authentication.