If we want to use the Expand Pane for a repeating grid, which of the following data sources can we use for the grid contents?

The use of a writable page as a data source for a repeating grid with an Expand Pane is appropriate because a writable page allows you to directly add, modify, or delete the data displayed in the grid. This direct interactivity is critical when you want users to be able to edit the information in the grid or add new entries without needing to first perform a separate action to update the underlying data.

On the other hand, while report definitions and declare pages do serve important functions in Pega, they are not suited for this purpose in the context of an Expand Pane in a repeating grid. Report definitions are typically used for presenting data in a read-only manner, while declare pages are mainly for defining data that can be referenced throughout the application, not for direct manipulation of data.

Thus, choosing a writable page ensures the grid can incorporate dynamic and editable content suitable for user interactions.