Which one of the following actions should be taken by a client to verify the entity that they received a certificate from is the entity that should be using the certificate?

Encrypting a message with the peer's public key (extracted from the certificate) and verifying the peer can return the decrypted plaintext proves the peer possesses the corresponding private key - only the legitimate key holder can perform the decryption, confirming the certificate rightfully belongs to that entity.

A client never has access to the CA's private key - clients use the CA's PUBLIC key from the trusted root certificate to decrypt and verify the certificate's digital signature, making this option both procedurally and cryptographically incorrect.

Comparing the issuer and subject fields only reveals certificate metadata - it does not prove the presenting entity actually controls the private key paired with the public key embedded in the certificate.

Phoning the CA is not part of any standard PKI validation mechanism - certificate validation is performed entirely through cryptographic operations, not out-of-band manual verification.