- All of the listed answers are correct
- An iOS app
- An Android app
Explanation: Similar to the desktop video Masthead, the mobile video Masthead is a native video-based ad format that only requires a YouTube video to create, and appears on the YouTube homepage on mobile and tablet devices. This includes Android and iOS mobile apps as well as m.youtube.com for smartphone and tablets.
Read more here: https://support.google.com/displayspecs/answer/6244544