The correct answer is:
B. It is an IPsec extension that forces remote VPN users to authenticate using their credentials (username and password).
Explanation:
eXtended Authentication (XAuth) is an IPsec extension that adds additional authentication for remote VPN users after the initial IPsec phase 1 and phase 2 negotiations. XAuth requires users to provide their credentials (username and password) in addition to the standard IPsec authentication, enhancing the security of the VPN connection.

Firewall authentication generally requires the DNS service to be enabled in the firewall policy to correctly resolve hostnames during the authentication process. If DNS is not allowed in the firewall policy, the FortiGate cannot resolve external domains, and as a result, the user may not be presented with the login prompt when attempting to access an external website.
References:
* FortiOS 7.4.1 Administration Guide: Firewall Authentication Configuration

For SSL VPN to function correctly between two FortiGate devices, the following settings are required:
* B. The server FortiGate requires a CA certificate to verify the client FortiGate certificate: The server FortiGate must have a Certificate Authority (CA) certificate installed to authenticate and verify the certificate presented by the client FortiGate device.
* C. The client FortiGate requires a client certificate signed by the CA on the server FortiGate: The client FortiGate must have a client certificate that is signed by the same CA that the server FortiGate uses for verification. This ensures a secure SSL VPN connection between the two devices.
The other options are not directly necessary for establishing SSL VPN:
* A. The client FortiGate requires the SSL VPN tunnel interface type to connect SSL VPN: This is incorrect as SSL VPN does not require a specific tunnel interface type; it typically uses an SSL VPN client profile.
* D. The client FortiGate requires a manually added route to remote subnets: While routing may be necessary, it is not specifically required for the SSL VPN functionality between two FortiGates.
References
* FortiOS 7.4.1 Administration Guide - Configuring SSL VPN, page 1203.
* FortiOS 7.4.1 Administration Guide - SSL VPN Authentication, page 1210.

In flow-based inspection mode, FortiGate sends a reset (RST) packet to the client instead of providing a replacement message, which causes the block message not to be displayed.

Change the csf setting on Local-FortiGate (root) to set fabric-object-unification default.
The CLI command set fabric-object-unification is only available on the root FortiGate. When set to local, global objects will not be synchronized to downstream devices in the Security Fabric. The default value is default.
Option A will not synchronise global fabric objects downstream.