The correct answer is 'The private key associated with the certificate has been compromised.' Revocation of an SSL certificate is a critical action typically performed to mitigate the risk associated with a compromised private key, as anyone with access to it could potentially decrypt or spoof secure communications. The other options, while associated with SSL certificates, do not directly relate to the immediate need for revocation. Certificate chains and SHA-256 hashes are components for ensuring the integrity and trustworthiness of certificates, but they do not constitute reasons for revocation. Certificate expiration is a natural lifecycle event and does not require revocation, as the certificate will become invalid at the expiration date.