A conversion is:

  • When someone reaches your landing page after clicking your ad or video
  • Any interaction with your ad that can be measured, like watching a video for a certain length of time
  • By definition when someone makes a purchase after clicking on your ad
  • An action defined as valuable to your business that someone takes after clicking on your ad

The correct answer is:

  • An action defined as valuable to your business that someone takes after clicking on your ad

Explanation: An action that’s counted when someone interacts with your ad (for example, clicks a text ad or views a video ad) and then takes an action that you’ve defined as valuable to your business, such as an online purchase or a call to your business from a mobile phone.

Read more here: https://support.google.com/partners/answer/6365