Which of the following is NOT a possible source of data for a microflow?

The correct choice highlights the fact that a page itself is not treated as a direct data source in a microflow. A microflow can receive input parameters, which may include data passed from a page, but it is the data contained within the page that is utilized, not the page as the data source itself.

Input parameters from a page can represent various types of data, such as objects or primitive types, but the page is essentially a user interface component and does not serve as a data source in the way that a database action, a REST call, or constants defined in the microflow do.

To clarify further about the other options: a database action retrieves or modifies data stored in the database, an object retrieved from a REST call brings external data into the microflow, and constants are predefined static values that can be used for various purposes during the execution of a microflow. Each of these options serves as a direct source or reference for the data utilized within a microflow, thus reinforcing the distinction that a page itself is not a direct source in this context.