The Internet Key Exchange version 2 (IKEv2) protocol simplifies the negotiation process compared to IKEv1. It is defined by a specific sequence of message exchanges to establish a secure IPsec tunnel.
The correct chronological order of the IKEv2 exchanges is:
IKE_SA_INIT (Initial Exchange):
This is the first exchange. It negotiates the security parameters for the IKE Security Association (IKE SA), sends nonces, and performs the Diffie-Hellman key exchange. At the end of this exchange, the communication is encrypted, but the peers are not yet authenticated.
IKE_AUTH (Authentication Exchange):
This is the second exchange. It authenticates the previous messages, exchanges identities and certificates (if used), and establishes the first Child SA (the actual IPsec Security Association used for data traffic).
CREATE_CHILD_SA (Subsequent Exchanges):
This exchange occurs after the IKE SA and the initial Child SA are established. It is used to create additional Child SAs (for different traffic selectors) or to perform re-keying for the IKE SA or existing Child SAs.
Why other options are incorrect:
A & B: Incorrect because CREATE_CHILD_SA cannot happen before the SA is initialized (IKE_SA_INIT) and authenticated (IKE_AUTH).
D: Incorrect because IKE_AUTH cannot occur before IKE_SA_INIT.
Therefore, the protocol flow is IKE_SA_INIT $\rightarrow$ IKE_AUTH $\rightarrow$ CREATE_CHILD_SA.

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."