The requirement is to keep the actual content of an email confidential and verifiable, which means the encryption has to be applied to the message itself, not merely to the connection carrying it.

S/MIME (Secure/Multipurpose Internet Mail Extensions) does exactly this: it uses public-key cryptography together with digital certificates to encrypt the message body and to attach a digital signature.

Encryption ensures only the intended recipient (holder of the matching private key) can read the content, while the signature lets the recipient verify the sender's identity and that the message was not altered.

IMAP is only a protocol for retrieving and organising messages stored on a mail server; it does not encrypt message content.

SSL (now TLS) secures the transport link while mail is in transit between client and server, but it does not protect the message once it is stored on a server, so the content is not encrypted end to end.

SMTP is the protocol that transfers mail between servers, and on its own it carries the message in plaintext, offering no content confidentiality.

Because only S/MIME encrypts and digitally signs the message body itself, the correct answer is S/MIME.