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

  • A) watches at least 25% of the video
  • B) watches the video until the last quartile
  • C) watches at least 30 seconds of the video or completes it
  • D) watches the video for five seconds or longer

The correct answer is:

C) watches at least 30 seconds of the video or completes it

Explanation: Viewers can choose to skip the video ad after 5 seconds. If they choose not to skip the video ad, the 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.

Read more here: https://support.google.com/displayspecs/answer/6055025?hl=en