- Desktop and mobile web
- Mobile web and mobile app
- Mobile app and mobile app interstitial
- YouTube and open exchange
The correct answer is: YouTube and open exchange
Explanation: YouTube and open exchange environments and inventory sources cannot run within a single line item. The environment can’t be changed once you send the proposal to the buyer for the first time.
Read more here: https://support.google.com/admanager/answer/6239628