The correct answer is Using multi factor authentication.

Using multi factor authentication requires users to present at least two independent forms of verification such as a password combined with a hardware token or a biometric. This approach greatly reduces the risk that a compromised password alone will allow access and so it provides the strongest protection for user authentication to a cloud hosted containerized application.

Using multi factor authentication can be enforced per user through identity providers and cloud IAM which preserves accountability and enables targeted revocation. It also defends against common attacks like phishing and credential stuffing which makes it the most robust choice here.

Using a personal VPN client can protect network traffic and restrict access to a private network but it does not by itself provide strong per user authentication. A VPN can be bypassed or misused if credentials or the endpoint are compromised.

Sharing a single set of account credentials among multiple users removes individual accountability and prevents per user authentication and revocation. It also makes it impossible to enforce individualized second factors and increases the blast radius of a credential compromise.

Using Identity Aware Proxy adds an access control layer that evaluates identity and context but it is not a substitute for requiring multiple authentication factors. IAP is typically used together with multi factor authentication rather than replacing it.

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