TrueView Video Discovery ads can run on:

A) The Display Network
B) The Search Network and the Display Network
C) YouTube and the Display Network
D) YouTube

The correct answer is:

C) YouTube and the Display Network

Explanation: TrueView Video Discovery ads appear on YouTube-related videos search results pages; the video opens on a YouTube watch or channel page, Video partner sites and apps on the GDN
Read more here: https://support.google.com/adwordspolicy/answer/2679940