A is incorrect because the app.yaml file does not include project-related information; it only contains configuration settings for the App Engine application. B is incorrect as the web-application.xml file, typically used for Java Servlet configurations, does not hold GCP project details. C is not applicable because App Engine deployments do not utilize Deployment Manager, which is for infrastructure deployment management. D is correct because the gcloud config list command displays the active configuration, including the project ID currently targeted by gcloud. This helps identify if the application was deployed to the intended project or elsewhere. For more details, refer to Google Cloud's documentation on App Engine deployments and gcloud app deploy reference.