Which BPMN object is used to visualize a set of related activities within a department?

The correct choice is Lane, which is a BPMN (Business Process Model and Notation) element specifically designed to represent activities within a particular organizational unit or department. Lanes are used to partition pools into subcategories, making it easier to see how various activities and tasks are related to specific roles or departments within the larger process. Each lane can contain its own set of tasks, events, and gateways that help illustrate the workflow and responsibilities of the department.

In contrast, a Pool serves as a high-level container for an entire process or organization, often representing different participants in a collaboration. A Task is a discrete unit of work that is performed within the BPMN diagram, but it does not provide a broader context of organizational structure. An Event indicates something that happens during the course of the process that can affect its state, like starting, ending, or interrupting a workflow, but does not specifically visualize the relationships among activities within a department. This makes Lane the most appropriate choice for visualizing the activities related to a specific department in a BPMN diagram.