A TrueView in-stream ad view is counted when a viewer:

A) lands on the YouTube page where the ad is running
B) engages with the ad, for example, by clicking a call to action
C) watches or skips the ad
D) watches the ad for the second time

The correct answer is:

B) engages with the ad, for example, by clicking a call to action

Explanation: YouTube video view count will be incremented when the viewer watches 30 seconds of the video ad (or the duration if it’s shorter than 30 seconds) or engages with your video, whichever comes first. Video interactions include clicks to visit your website, call-to-action overlays (CTAs), cards, and companion banners. If view counts on YouTube are a concern, it’s a good idea to make videos at least 12 seconds long. YouTube analytics doesn’t track views less than 10 seconds
Read more here: https://support.google.com/displayspecs/answer/6055025