To send a PGP encrypted message, which piece of information from the recipient must the sender have before encrypting the message?

The recipient's private key is never shared with anyone; sharing it would completely compromise the recipient's security and violate the fundamental principle of asymmetric cryptography.

PGP uses asymmetric (public-key) cryptography where each user has a key pair - a public key that is freely shared and a private key that is kept secret. To send an encrypted message, the sender encrypts the message using the recipient's public key; only the recipient, who holds the corresponding private key, can decrypt it. This ensures confidentiality without requiring the sender to ever know the recipient's private key.

There is no 'master encryption key' in PGP; the protocol relies on individual public-private key pairs, not a centralized shared master key.

The sender's public key is used by recipients to verify the sender's digital signature, not to encrypt messages destined for a different recipient.