If a message is encrypted using a public key, which key is needed to decrypt it?

When a message is encrypted using a public key, the only key that can decrypt it is the corresponding private key. This mechanism is fundamental to asymmetric encryption, where two different keys, a public key and a private key, are used. The public key can be freely distributed and used to encrypt messages, ensuring that only the individual who possesses the matching private key can decrypt the message.

Using the private key for decryption maintains the confidentiality and integrity of the message, as it allows only the intended recipient to access the content. Therefore, the requirement to use their private key is essential for maintaining security in this encryption scheme.

Other options do not reliably achieve decryption of the message encrypted with a public key, as they would either not correspond to the necessary key types or would introduce security vulnerabilities.