Final answer:
Use Case Diagram (C) is the most suitable for describing how users interact with existing systems, as it illustrates the relationships between users (actors) and the system's functionalities.
Step-by-step explanation:
The most appropriate diagram to describe how users do their jobs with the existing systems is C. Use Case Diagram. Use case diagrams are part of the Unified Modeling Language (UML) and are used to represent the interactions between users (actors) and use cases, where a use case is a sequence of actions that provide a measurable value to an actor.
A use case diagram is ideal for showing the relationships between actors and the systems they interact with, clearly delineating the boundaries of the system, the roles of each actor, and how they interact with the system functionalities. This helps in visualizing the functional requirements of a system from the user's perspective, making it an excellent tool for capturing user-system interactions.