Which of the following objects in an application must be stored within a folder?

The process model is the correct choice because, in Appian, process models must always be stored within a folder. This organizational structure helps manage and categorize the various components of an application effectively. Folders in Appian serve as containers that help maintain a clean and navigable application structure, especially when dealing with multiple process models, which can be complex and numerous.

In contrast, constants, rules, and data types do not require a specific folder for their storage. Constants can exist independently and can be used throughout the application without being confined to a folder structure. Similarly, rules and data types have more flexible storage options, often allowing for a broader range of organizational strategies that do not mandate folder placement. The requirement for process models to reside in a folder reflects their role as central components of application workflow and underscores the importance of organization in managing these workflows.