From the exhibits:

The Web Filter profile is configured with Feature set = Flow-based.

The Firewall policy is configured with Inspection mode = Proxy-based and has Web Filter enabled.

In FortiOS 7.6, security profiles that have a feature set selection (Flow-based vs Proxy-based) must match the inspection mode used by the firewall policy. If the profile’s feature set does not match the policy’s inspection mode, the profile behavior will not align with what the administrator expects (and in many cases FortiOS will prevent correct use/selection, or the feature behavior will not apply as intended).

That mismatch explains why the configured URL filter entry for www.facebook.com (set to Monitor) is not producing the expected result, and instead the session is being evaluated by category rating and blocked (shown as Malicious Websites on the FortiGuard block page).

Why the other options are not the best fit:

A: A web rating override is not shown in the exhibits, and nothing indicates an override misconfiguration.

C: While the policy inspection mode could be changed, the root cause shown is the profile feature set mismatch (profile is Flow-based).

D: The URL filter action shown is Monitor, which would not produce a block page by itself.