Who can a subscriber share their private signing key with?

The option indicating that a subscriber cannot share their private signing key with anyone is correct because the integrity and security of public key infrastructure (PKI) depend heavily on the confidentiality of private keys. Private signing keys are unique to each subscriber and are used to create digital signatures, which authenticate the identity of the signer and ensure the integrity of the signed data.

If a subscriber were to share their private signing key, it would compromise the security model of PKI, as anyone with access to that key would be able to create fraudulent signatures in the subscriber's name. The primary design principle behind PKI is to maintain a clear separation between public and private keys; the private key must remain a secret to protect the authenticity of the digital signatures it generates.

The other options would imply scenarios where sharing the private key might be seen as acceptable, which would not align with best practices in PKI security and could lead to significant security breaches.