Final answer:
Message extensions, callout extensions, and sitelink extensions can serve automatically, while structured snippets cannot.
Step-by-step explanation:
The ad extensions that can serve automatically are:
- Message extensions: Allow users to send text messages directly from the ad.
- Callout extensions: Add additional text to your ad, highlighting specific information or promotions.
- Sitelink extensions: Provide additional links to specific pages on your website.
Structured snippets cannot serve automatically, as they require manual input and are not clickable.