___________ are optional, clickable thumbnail images that appear next to TrueView InStream ads on YouTube.

A) Companion banners
B) TrueView Video Discovery ads
C) YouTube Mastheads
D) Companion Mastheads

The correct answer is:

A) Companion banners

Explanation: A clickable thumbnail image that accompanies a TrueView in-stream ad. On a YouTube page, it appears next to the ad, in the top right corner (on a desktop computer). Companion banners are optional. They’re also displayed only in certain cases, for example, in the context of a YouTube watch page. They don’t show on embedded players, connected TVs, or game consoles.


Read more here: https://support.google.com/partners/answer/6293542?hl=en