TrueView in-stream ads and Video Discovery ads appear, respectively:

A) before videos and as clickable thumbnails
B) as clickable thumbnails and before videos
C) to the left and right of videos
D) at the top and bottom of videos

The correct answer is:

A) before videos and as clickable thumbnails

Explanation: TrueView Video Discovery ads include the following old formats:

YouTube Promoted Videos overlay
Promoted videos suggestions and related videos
TrueView in-stream videos can appear on YouTube watch pages and on video partner sites and apps in the Google Display Network.
Read more here: https://support.google.com/adwords/answer/2375464