BFD (Bidirectional Forwarding Detection) is a protocol that provides fast failure detection for BGP by sending periodic messages to verify the connectivity between two peers1. BFD can be enabled on both connected FortiGate devices by using the command set bfd enable under the BGP configuration2. References:
= Technical Tip : FortiGate BFD implementation and examples ..., Configure BGP | FortiGate / FortiOS
7.0.2 - Fortinet Documentation

The "single-source" option ensures that only one remote site is connected at any time, which aligns with the requirement in the question. This option prevents multiple VPN tunnels from being established between the same source and destination networks, and allows only the most recent tunnel to be active. This can be useful for scenarios where multiple remote sites have the same IP address range, as shown in the exhibit. Reference := Fortinet Enterprise Firewall Study Guide for FortiOS 7.2, page 142.

* Option A is correct because to allow SSH, you need to specify SSH in the Service field of the policy configuration. This is because the Service field determines which types of traffic are allowed by the policy1. By default, the Service field is set to App Default, which means that the policy will use the default ports defined by the applications. However, SSH is not one of the default applications, so you need to specify it manually or create a custom service for it2.
* Option B is incorrect because configuring port 22 in the Protocol Options field is not enough to allow SSH. The Protocol Options field allows you to customize the protocol inspection and anomaly protection settings for the policy3. However, this field does not override the Service field, which still needs to match the traffic type.
* Option C is incorrect because including SSH in the Application field is not enough to allow SSH. The Application field allows you to filter the traffic based on the application signatures and categories4.
However, this field does not override the Service field, which still needs to match the traffic type.
* Option D is incorrect because selecting an application control profile corresponding to SSH in the Security Profiles section is not enough to allow SSH. The Security Profiles section allows you to apply various security features to the traffic, such as antivirus, web filtering, IPS, etc. However, this section does not override the Service field, which still needs to match the traffic type. References: =
* 1: Firewall policies
* 2: Services
* 3: Protocol options profiles
* 4: Application control

* Option A is correct because the address object on the tool FortiGate will not be synchronized with the downstream devices if it has fabric-object set to disable. This option controls whether the address object is shared with other FortiGate devices in the Security Fabric or not1.
* Option C is correct because the downstream FortiGate will not receive the address object from the tool FortiGate if it has fabric-object-unification set to local. This option controls whether the downstream FortiGate uses the address objects from the root FortiGate or its own local address objects2.
* Option B is incorrect because the root FortiGate has configuration-sync set to enable by default, which means that it will synchronize the address objects with the downstream devices unless they are disabled by the fabric-object option3.
* Option D is incorrect because the downstream FortiGate has configuration-sync set to local by default, which means that it will receive the address objects from the root FortiGate unless they are overridden by the fabric-object-unification option4. References: =
* 1: Group address objects synchronized from FortiManager5
* 2: Security Fabric address object unification6
* 3: Configuration synchronization7
* 4: Configuration synchronization7
* : Security Fabric - Fortinet Documentation

Fail-open is a feature that allows traffic to pass through the IPS sensor without inspection when the sensor fails or is overloaded. If fail-open is set to disable, traffic will be dropped in such scenarios1. Reference: = IPS | FortiGate / FortiOS 7.2.3 - Fortinet Documentation