Answer:
The correct option is c. clear-signed data.
Step-by-step explanation:
Clear-signed data enables a digital signature to be created by encrypting the message digest of the material to be signed using the signer's private key. But only the digital signature is encoded with base64 in this scenario. As a result, recipients who do not have access to S/MIME can see the content of the message but the message signature cannot be verified by them.
Based on the explanation above, the correct option is therefore c. clear-signed data.