With TrueView in-stream ads, an advertiser pays:

A) on a cost-per-thousand-impressions (CPM) basis
B) when the viewer watches at least 30 seconds of the video or completes it
C) only when the viewer clicks the video
D) when the viewer watches five seconds of the video

The correct answer is:

B) when the viewer watches at least 30 seconds of the video or completes it

Explanation: In-stream ads run before, during, or after videos on YouTube or on video partner sites and apps. They give viewers the option to skip the ad after 5 seconds. These ads help you drive awareness for your brand and products while people are watching a video. With TrueView ads, you pay when a viewer watches 30 seconds of your video (or the duration if it’s shorter than 30 seconds) or interacts with your video, whichever comes first. This means that you will only pay when someone chooses to interact with your ad.

Read more here: https://support.google.com/adwords/answer/2383007?hl=en