• Enter the creative sizes as comma-separated values, select the corresponding creative sizes, and click “Apply”
• Enter the creative size and click “Apply”
• Enter one creative size, select the creative for that size, and click “Apply”
• Select all creative sizes and click “Apply”

The correct answer is: Enter one creative size, select the creative for that size, and click “Apply”

Explanation: When using creative dimension filtering, the system requires a specific size to correctly generate a dynamic preview. Entering one creative size ensures the platform knows exactly which dimensions to render. After filtering by size, you must select the corresponding creative so the preview tool loads the correct asset. Finally, clicking “Apply” confirms and activates the filter settings. Without applying the selection, the system will not process the changes, and the preview may not generate properly. These steps ensure accurate rendering of the intended creative variation.

Read more here: https://support.google.com/richmedia/answer/6023632