A click on a companion banner:

A) All of the listed answers are correct
B) counts as a view even if the person hasn’t watched 30 seconds of the ad
C) can direct to a YouTube channel
D) can direct to an external URL

The correct answer is:

A) All of the listed answers are correct

Explanation: 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.

A click on a companion banner: can direct to an external URL or a YouTube channel
counts as a view, even if the viewer hasn’t watched 30 seconds of the ad

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