Which of the following is an example of an architect's instrument of service?

Project drawings are indeed a prime example of an architect's instrument of service because they represent the tangible outcomes of an architect's design process and professional expertise. These drawings serve as critical documentation that conveys the architect’s vision, specifications, and the details necessary for construction. They include various types of drawings, such as site plans, floor plans, elevations, and sections, which collectively help ensure that the project is built according to the architect’s design intent.

In contrast, client correspondence, material invoices, and budget estimates, while important in the overall management of a project, do not directly embody the architectural design or work produced by the architect. Instead, they pertain to communication and financial aspects of the project rather than the direct services provided by the architect in shaping the built environment.