From the Ad Server drop-down list, select which ad server you are using: OpenX, DFP, or MoPub. If you choose DFP or MoPub, the Response Format field is removed from the form.
Note that if you are generating a DFP JavaScript tag, you must be using the latest Google MobileAds SDK - DFP integration (NOT Admob). You must also be using the DFP UI (NOT Admob) ad server to serve ads.