Before you run the gcloud compute instances list command, you need to do two things: authenticate with your user account and set the default project for gcloud CLI.
To authenticate with your user account, you need to run gcloud auth login, enter your login credentials in the dialog window, and paste the received login token to gcloud CLI. This will authorize the gcloud CLI to access Google Cloud resources on your behalf1.
To set the default project for gcloud CLI, you need to run gcloud config set project $my_project, where
$my_project is the ID of the project that contains the instances you want to list. This will save you from having to specify the project flag for every gcloud command2.
Option B is not recommended, because using a service account key increases the risk of credential leakage and misuse. It is also not necessary, because you can use your user account to authenticate to the gcloud CLI3.
Option C is not correct, because there is no such thing as a Cloud Identity user account key. Cloud Identity is a service that provides identity and access management for Google Cloud users and groups4. Option D is not required, because the gcloud compute instances list command does not depend on the default zone. You can list instances from all zones or filter by a specific zone using the --filter flag.
References:
* 1: https://cloud.google.com/sdk/docs/authorizing
* 2: https://cloud.google.com/sdk/gcloud/reference/config/set
* 3: https://cloud.google.com/iam/docs/best-practices-for-managing-service-account-keys
* 4: https://cloud.google.com/identity/docs/overview
* : https://cloud.google.com/sdk/gcloud/reference/compute/instances/list