A TrueView Video discovery ad needs to be:

A) There isn’t a time limit
B) about 2 minutes
C) less than 30 seconds
D) more than 30 seconds

The correct answer is:

A) There isn’t a time limit

Explanation: Your ad consists of a thumbnail image from your video with some text. While the exact size and appearance of the ad may vary depending on where it appears, video discovery ads always invite people to click to watch the video. The video then plays on the YouTube Watch page or on your channel page. You’ll be charged only when viewers choose to watch your ad by clicking the thumbnail.

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