To protect the sensitive data stored in Vault, what key is used to encrypt the...

Comprehensive and Detailed In-Depth Explanation:

Vault encrypts all data before writing it to the storage backend using an encryption key within its cryptographic barrier. This key, stored in a keyring, is itself encrypted by the master key (split into unseal keys). The recovery key (A) is for emergency recovery, not data encryption. Unseal keys (C) unlock the master key, not encrypt data directly. The root key (D) isn’t a term used in Vault’s encryption flow; the master key is the closest analog, but it protects the encryption key, not the data itself. The architecture docs clarify the encryption key’s role.