The correct answers are B and C.
The study guide has a section titled "Not Verified Status on the Collector Agent" and states:
"The collector agent cannot verify if the user is still logged in" and lists these common causes:
"A firewall is blocking traffic to port 139 and 445"
"The workstation remote registry service is not running"
The guide also explains the verification method:
"For WMI polling mode, the collector agent checks the WMI service. For all the other modes, the collector agent checks the HKEY_USERS hive through remote registry services." If the workstation does not respond to these checks, the status can become not verified An additional requirements slide in the same study guide confirms:
"TCP ports 139 and 445 must be open between the collector agent and all workstations"
"Remote registry service must be up and running on each workstation"
Why the other options are wrong:
A is wrong because the study guide mentions a workstation coming out of hibernate mode under a different problem: "No Internet After IP Address Change", not as a common cause of Not Verified status D is wrong because DNS resolution issues are also discussed under the IP address change scenario, where the collector agent uses DNS to resolve the workstation name after an IP change. That is separate from the Not Verified causes listed for this log message So the verified answers are: B, C.

References:
FortiOS Admin Guide: OSPF, Debug Outputs

The debug command diagnose debug application fssod -1 reveals the internal processing of the FortiGate Single Sign-On daemon.
* Option D (Agentless Polling): The output shows event_id=4768. Event ID 4768 (Kerberos TGT Request) is a Windows Event Log entry. The presence of specific Event IDs in the fssod debug, rather than a generic logon notification, indicates that the system is reading (polling) the Security Event Logs from the Domain Controller. This is characteristic of Agentless Polling Mode (or Collector Agent Polling Mode), where the FortiGate or Collector scrapes logs. In contrast, DC Agent mode intercepts logon calls directly and would typically provide more complete information, including the workstation name.
* Option A (Verification): Crucially, the output shows workstation=,, indicating the workstation name field is empty. In Polling Mode, certain Event IDs (like 4768) often do not contain the source workstation's hostname. Without the workstation name, the FortiGate (or Collector) cannot perform a workstation check (WMI/Registry poll) to verify if the user is still logged in. It essentially has to rely on the "dead entry timeout" because active verification is impossible without the target machine's name.
Option B is incorrect because DC Agents reliably capture workstation names. Option C is incorrect because the system cannot poll a workstation it cannot identify.