These resources, which were previously accessible, are now consistently unavailable for these users. There are no reports of general network slowdown or inaccessibility of other services. What is the most likely cause of this issue?

A$.$Hardware malfunction affecting only specific websites and documents

B$.$Network congestion affecting only certain internal resources

C$.$Blocked content due to recent security policy updates or content filtering changes

D$.$Server downtime affecting only specific internal resources