Which of the following functional requirements is most appropriate to implement with an Activity rule rather than a data transform?

Implementing parsing data from an external system is best suited for an Activity rule due to the need for more complex and dynamic control over the data manipulation process. Activities provide a scripting environment where developers can utilize various steps, including decision points, looping, and invoking other methods—capabilities that are crucial for tasks like parsing.

When handling data from an external source, there may be complex logic or conditions that need to be evaluated based on the structure of the incoming data. Activities facilitate this kind of nuanced programming, allowing for the incorporation of error handling and iterative processing, which are often necessary when dealing with external data formats.

In contrast, data transforms are better suited for simpler operations, such as initializing property values of a case instance or updating properties from one page to another. These tasks generally require straightforward field mapping or property assignments without the need for the advanced control logic that Activities offer. Copying a page into a page list property also falls into this category of basic property manipulation easily handled by data transforms.

Therefore, the choice of an Activity for parsing data from an external system reflects its requirement for complex processing, validating why this option is the most appropriate for such a functional requirement.