In which of the following scenarios is PKI LEAST hardened?

Posting the CRL to a publicly accessible location is the standard, expected behavior and is required for relying parties to check certificate revocation status.

Using symmetric keys for time offset recording is an unusual design but does not directly undermine the core certificate issuance and validation trust chain.

PKI security depends entirely on the trustworthiness of the CA certificates in the client trust store. A malicious CA certificate installed on all clients allows an attacker to issue fraudulent certificates for any domain that browsers and applications will accept as legitimate, enabling undetected man-in-the-middle attacks across the entire infrastructure - this is the most severe possible compromise of a PKI deployment.

Public keys are intentionally public by design; unauthorized access to them causes no security harm because they carry no secret information.