Which option would you use if you wanted to modify existing logic that has already been developed?

The choice of Workflow is appropriate when you want to modify existing logic that has been developed in ServiceNow. Workflows are visual representations of automated processes that consist of various activities and transitions. They allow users to outline the sequence of tasks and how they interact, making it easier to manage complex processes within the ServiceNow platform.

When existing workflows require changes—whether it's to add new tasks, modify existing conditions, or adjust paths through the workflow—the workflow editor provides the necessary tools to make these adjustments. The visual interface simplifies understanding the logic flow and interactions of different components, allowing for straightforward modifications without needing to delve into underlying scripts or coding details.

In contrast, while Flow Designer offers automation capabilities, it is more suited for creating new flows rather than modifying existing workflows. Script Includes are reusable server-side scripts that assist with encapsulating and executing code, but they don't directly represent or manage the graphical flow of logic like workflows do. Instance Designer, which deals with instance-specific configurations, does not focus on workflow modification. Thus, Workflow stands out as the correct option for modifying existing logic.