• The best performing placement in Campaign Manager
• The best performing creative in Campaign Manager
• The best performing columns in the feed
• The best performing rows in the feed

The correct answer is: The best performing rows in the feed

Explanation: “Open: Open optimization selects the highest performing row on any given impression. Use this option to optimize your feed when you’re not using other rules or dynamic strategies.”

Read more here:https://support.google.com/richmedia/answer/2871761