The correct answer is A because the simplest and fastest way to enable an API for your project is by using the Google Cloud Console. This method is straightforward and doesn't require additional permissions beyond those needed to access the console.

• Option B is incorrect because assigning the Pub/Sub Admin role to a service account doesn't grant the ability to enable APIs. Enabling an API requires specific IAM permissions (servicemanagement.services.bind and serviceusage.services.enable) that are not covered by the Pub/Sub Admin role.
• Option C is incorrect because all applications, regardless of where they're hosted (including App Engine), require the relevant APIs to be enabled before they can be used.
• Option D is incorrect because APIs are not automatically enabled upon first use; they must be explicitly enabled before they can be accessed by any application.