Which of the following means that the receiver must be able to prove that the…

Message-level security decomposes into distinct guarantees: confidentiality (privacy) protects the message content from unauthorized reading, integrity ensures the content was not altered in transit, authentication confirms a claimed identity at the time of communication, and non-repudiation additionally gives the receiver durable, independently verifiable proof — typically via a digital signature — that a specific sender originated a message, so that sender cannot later deny sending it.

The stem asks for the service where the receiver must be able to PROVE, after the fact, that a message came from a particular sender. That is exactly non-repudiation: a digital signature lets the receiver (or a third party) verify the message's origin independently of the sender's cooperation, closing off any later denial by that sender.

Contrasting the near-miss options:

• Integrity confirms the content itself was not altered in transit — it says nothing about proving who sent it.

• Privacy (confidentiality) keeps the content secret from outsiders — again unrelated to proving the sender's identity.

• Authentication verifies identity at the time of communication, but does not by itself leave the receiver holding durable proof usable later if the sender disputes having sent the message — that stronger, persistent guarantee is specifically non-repudiation.