Final answer:
Project managers use a portfolio management system to oversee multiple projects, not just a single project plan, Gantt chart, risk assessment, or budget.
Step-by-step explanation:
A project manager who is overseeing multiple projects might use a portfolio management system or methodology to effectively manage them. Although the options provided include project plan, Gantt chart, risk assessment, and budget, these tools are generally used for managing individual projects. Specifically, a project plan is a formal document that outlines the execution and control of a single project, a Gantt chart is a visual timeline used to schedule and track tasks within a project, a risk assessment helps identify potential problems that could negatively impact a project, and a budget entails the financial aspects of a project.
Therefore, while the provided choices are closely associated with project management, the actual term that encompasses the control and administration of multiple projects is portfolio management. Through portfolio management, a project manager can allocate resources, prioritize projects, and ensure that the organization's strategic objectives are being met across all projects.