The direct protection of integrity is provided by Digital Signature.
A digital signature is a cryptographic mechanism used to protect the integrity of a message or document. It provides a way to ensure that the message or document has not been altered in transit and that the sender is authentic. The digital signature is created using the sender's private key and verified using the sender's public key.
Asymmetric encryption, symmetric encryption, and digital envelope provide confidentiality by encrypting the message or document. While confidentiality is important for maintaining privacy, it does not directly protect the integrity of the message or document.
Therefore, the correct answer is option 2: Digital signature.