For auto-tagging, Google Ads includes gclid= parameter in the destination URL.
The correct answer is: gclid=
When you enable auto–tagging, a parameter called gclid is added to your landing page URL when a user clicks over to your site from an ad. For example, if your site is www.example.com, when a user clicks on your ad it appears in the address bar as: www.example.com/?gclid=123xyz.
If you don’t see the gclid parameter appended to your URLs after the page is fully loaded, then it’s likely that your website isn’t configured to handle query parameters in a way that lets auto-tagging work.
Read more here: https://support.google.com/analytics/answer/2938246