Which of the following data types cannot be referenced using a Constant?

The reason User is the correct choice for a data type that cannot be referenced using a Constant is that Appian's Constant feature is designed to store values that remain constant throughout the application. While text, number, and date and time data types can represent fixed values that do not change during the execution of an application, User data types are dynamic and associated with specific user accounts within Appian.

User data types require interaction with Appian's security model and potentially change, as users can be added, modified, or deleted. Therefore, it is crucial to handle User data in a way that accommodates these changes, making it unsuitable for use as a Constant.

In contrast, text, number, and date and time can hold specific values that are static and can be referenced throughout an application without concern for change, making them well-suited for use as Constants.