The correct option is Login name.

A login name is primarily an identifier for an account and it does not constitute proof of who is operating that account. Usernames are often visible, predictable, or reused and they point to an account record rather than providing evidence of possession, knowledge, or inherence.

Proving identity requires authenticators such as secrets, tokens, or biometrics because those provide evidence that a claimant controls or is associated with the account. A login name alone should not be relied upon to prove identity.

Service account key is a credential used by applications or services to authenticate and authorize actions, so it functions as an authenticator rather than merely an identifier.

Retinal scan is a biometric method that verifies identity based on inherence and it is used to prove who someone is rather than to identify an account.

Hardware token is a possession based authenticator that provides proof of control and is used in multi factor authentication to demonstrate identity rather than to serve as an account identifier.

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