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 must never be shared - it is the secret key used only by the recipient to decrypt incoming messages.

PGP uses asymmetric (public-key) cryptography where each user has a key pair - a public key shared openly and a private key kept secret. To encrypt a message for a recipient, the sender must obtain and use the recipient's public key. The encrypted message can then only be decrypted by the corresponding private key held solely by the recipient.

There is no 'master encryption key' in PGP's key model - PGP relies on individual user key pairs.

The sender's public key is used by others to send encrypted messages to the sender, not by the sender to encrypt outgoing messages.