Why is the error occurring?

An organization is implementing URL blocking using Cisco Umbrella. The users are able to go to some sites but other sites are not accessible due to an error.

Why is the error occurring?
A . Client computers do not have the Cisco Umbrella Root CA certificate installed.
B . IP-Layer Enforcement is not configured.
C . Client computers do not have an SSL certificate deployed from an internal CA server.
D . Intelligent proxy and SSL decryption is disabled in the policy

Answer: A

Explanation:

Reference: https://docs.umbrella.com/deployment-umbrella/docs/rebrand-cisco-certificate-import-information