When Should an Advertiser Use an App Extension Instead of an App

When should an advertiser use an app extension instead of an app promotion ad?

A) With keywords intended to drive app download, but still give the option to visit the mobile website
B) With keywords intended to direct users to the mobile website, but still give the option to download the app
C) To re-engage with users who have already downloaded the app
D) To run your ad on both Search and Display