The reported symptom-users unable to access any websites despite no explicit blocks in the profile-points to systemic connectivity or configuration issues rather than specific URL filtering rules.
Option B (SSL/TLS Inspection): When Deep Inspection is enabled, the FortiGate acts as a Man-in-the-Middle (MitM) and re-signs server certificates using its own CA. If the clients (browsers) do not trust this CA (i.e., the certificate is not installed in their Trusted Root store), they will reject the connection with certificate errors, effectively preventing access to all HTTPS websites.
Option D (DNS): Web browsing relies on DNS resolution. If the configured DNS server is unreachable or failing, the FortiGate (or the client) cannot resolve FQDNs to IP addresses. Consequently, browsers will fail to load any page, resulting in a total loss of web access.
Option E (License): If the FortiGuard Web Filtering license expires, the FortiGate can no longer query the FortiGuard Distribution Network (FDN) for ratings. By default, or if the allow-when-rating-error setting is disabled (a common security practice), the FortiGate will block all web traffic that it cannot rate, often displaying a "Web Filter Service Error" or invalid license page.
Option A is incorrect because clearing the cache only increases latency, it does not block traffic. Option C is incorrect because webfilter-force-off is typically used to disable the service (often allowing traffic to bypass checks if the service is down), rather than blocking it.