The best option is to instruct the external consultant to generate an SSH key pair, and request the public key from the consultant. Then, add the public key to the instance yourself, and have the consultant access the instance through SSH with their private key. This way, you can grant the consultant access to the instance without requiring a Google account or exposing the instance's public IP address. This option also follows the best practice of using user-managed SSH keys instead of service account keys for SSH access1.
Option A is not feasible because the external consultant does not have a Google account, and therefore cannot use Identity-Aware Proxy (IAP) to access the instance. IAP requires the user to authenticate with a Google account and have the appropriate IAM permissions to access the instance2. Option B is not secure because it exposes the instance's public IP address, which can increase the risk of unauthorized access or attacks. Option D is not correct because it reverses the roles of the public and private keys. The public key should be added to the instance, and the private key should be kept by the consultant. Sharing the private key with anyone else can compromise the security of the SSH connection3.
Reference:
1: https://cloud.google.com/compute/docs/instances/adding-removing-ssh-keys
2: https://cloud.google.com/iap/docs/using-tcp-forwarding
3: https://cloud.google.com/compute/docs/instances/connecting-advanced#sshbetweeninstances