If your client wants to pay only when someone views an ad, you should use:

A) YouTube homepage ads
B) Text ads
C) Viewable cost-per-thousand-impressions (vCPM) bidding
D) TrueView in-stream ads

The correct answer is:

D) TrueView in-stream ads

Explanation: Use this format when you have video content you’d like to promote before other videos on YouTube and across the Google Display Network. Your video ad plays before, during, or after other videos. After 5 seconds, the viewer has an option to skip the ad. 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.

Viewers see videos they’re curious about, and you get more views from an audience you know is interested. Because you choose what you want to pay for a view, you get the right audience at the right price. Unlike cost-per-impression (CPM) pricing, you won’t need to pay every time your ad is shown.

Read more here: https://support.google.com/adwords/answer/2375464