The correct answer is A because the VM needs to be granted permissions using a service account to communicate with Cloud Pub/Sub. This involves creating a new service account with the necessary Cloud Pub/Sub access and associating it with the instance.

• Option B is incorrect because Cloud Pub/Sub does not support IP whitelisting; firewalls are applicable only to Compute Engines.
• Option C is not the best choice as service accounts are designed for service-to-service communication, handling OAuth authentication internally.
• Option D is incorrect because if the service account has no permissions, the instance will not be able to communicate with Cloud Pub/Sub, regardless of the access scope settings.

For more details, refer to the GCP documentation on Service Account Permissions.