Option c is the way to go. Up to the app developer to see how to monetize the app. Bonus points if the end user can be identified. E.g. if the app can pass an

Option c is the way to go. Up to the app developer to see how to monetize the app. Bonus points if the end user can be identified. E.g. if the app can pass an opaque token to the web service. Reporting / billing from the web service provider groups usage by token.