While troubleshooting a client application connecting to the network, the security administrator notices the following error: Certificate is not valid. Which of the following is the BEST way to check

Public Key Infrastructure (PKI) is the entire framework for creating, managing, distributing, and revoking digital certificates, not a direct tool for checking a single certificate's validity.

A Certificate Revocation List (CRL) is a list maintained by a Certificate Authority (CA) that contains certificates which have been revoked before their scheduled expiration date. By checking if the client application's certificate appears on the CRL, an administrator can confirm its invalidity due to revocation, addressing the 'Certificate is not valid' error.

A Certificate Signing Request (CSR) is a formatted message sent to a Certificate Authority (CA) to request a new digital certificate, not a method for verifying the validity of an existing one.

Internet Protocol Security (IPSec) is a suite of protocols used to secure IP communications, often utilizing certificates for authentication, but it is not a mechanism for checking certificate validity itself.