Which of the following can only be provided by asymmetric encryption?

Information privacy (confidentiality) can be provided by both symmetric and asymmetric encryption through encryption of data.

Data availability is not directly a function of encryption; encryption secures data but doesn't inherently guarantee its presence or accessibility.

256-bit key length is a measure of key strength and can be used with both symmetric (e.g., AES-256) and asymmetric (e.g., elliptic curve cryptography often has equivalent strengths) encryption algorithms.

Asymmetric encryption uses a public/private key pair where only the private key can create a digital signature that can be verified by the corresponding public key, thereby providing cryptographic proof of the sender's identity and preventing them from denying having sent a message (nonrepudiation). Symmetric encryption cannot offer nonrepudiation because both sender and receiver share the same secret key, making it impossible to definitively prove who generated a message.