A FortiLink interface must be enabled on FortiGate (A): To manage a FortiSwitch stack, a dedicated FortiLink interface on the FortiGate is required. This interface is used to manage the communication between FortiGate and the FortiSwitch stack, enabling centralized control and configuration of the switches directly from the FortiGate.
The switch controller feature must be enabled on FortiGate (B): Enabling the switch controller feature on FortiGate allows it to manage connected FortiSwitch units. This feature provides tools and interfaces on the FortiGate for overseeing FortiSwitch configurations, monitoring switch status, and managing network policies across the stack.

The provided debug output indicates that the FortiSwitch is sending FortiLink heartbeats to the FortiGate and is currently waiting to join the stack group. Here's a breakdown of the relevant lines:
Line 1: Shows the date, time, elapsed time since boot, and process ID for the FortiLink event handler.
573s:160ms: 74us translates to roughly 573 seconds, 160 milliseconds, and 74 microseconds since uptime.
Event 101: This indicates the FortiSwitch is in a "wait join" state (FL_STATE_WAIT_JOIN). This means it's discovered by the FortiGate and is awaiting further instructions to join the FortiLink stack group.
switchname S424DPTF20000029: This displays the serial number of the FortiSwitch.
flags 0x401: The specific flag meaning might depend on the FortiSwitch model and version, but it likely indicates general communication between the switch and FortiGate.
Lines 2 and onward: These lines show subsequent events with similar timestamps, suggesting a regular heartbeat interval. There are also instances of the FortiSwitch sending packets to the FortiGate (indicated by pkt-sent).
Why the Other Options Are Less Likely:
C . FortiSwitch is discovered and authorized by FortiGate. While discovery might have happened before these lines, the "wait join" state suggests authorization hasn't necessarily completed yet.
D . FortiSwitch is ready to push its new hostname to FortiGate. There's no explicit indication of hostname changes in this excerpt. The focus is on joining the stack group.
In Summary:
The key point is the "FL_STATE_WAIT_JOIN" state, which signifies the FortiSwitch is ready to be fully integrated but is waiting for further commands from the FortiGate to complete the process.

VLAN assignments on FortiSwitch ports must follow certain rules and guidelines to ensure network integrity and proper traffic segregation:
Only Assign One Native VLAN on a Port (C):
Native VLAN Configuration: Each switch port can have only one native VLAN. The native VLAN carries untagged traffic for that port. If the port receives untagged frames, they are assumed to belong to the native VLAN.
Importance of Singular Native VLAN: This is crucial for preventing VLAN hopping attacks and ensures clear and secure VLAN demarcation on each port.
Assign Untagged VLANs Using FortiGate CLI (D):
CLI Configuration: Untagged VLANs, often equivalent to the native VLAN, can be assigned through the FortiGate CLI when managing a FortiSwitch via FortiLink. This allows for central management and configuration of VLANs across connected switches.
Operational Efficiency: Using the CLI ensures that VLAN settings are applied uniformly, reducing the likelihood of misconfigurations that might occur when managing VLANs individually on each switch.
Reference:
For detailed instructions and best practices on VLAN configuration on FortiSwitch, refer to the FortiSwitch administration guide available on: Fortinet Product Documentation