• Add 10 rows and 10 columns with an audience ID in each row and column
• Add 10 rows with a different audience ID in each row
• Add 10 columns with a different audience ID in each column
• Add 10 comma-separated audience IDs into one cell

The correct answer is: Add 10 comma-separated audience IDs into one cell

Explanation: If you are using multiple values in a single targeting cell, then they should be comma-separated. If you want to enter more than one ID in the same column, use a comma to separate IDs.

Read more here: https://support.google.com/richmedia/answer/7159822