What do you call a diagram that illustrates a system of vectors?

The correct choice, a diagram that illustrates a system of vectors, is known as a vector diagram. Vector diagrams are specifically designed to represent vectors, showing both their magnitude and direction. These diagrams are essential in physics and engineering to visualize how different forces or vector quantities interact with one another. By employing arrows of varying lengths to represent different magnitudes and pointing in the appropriate directions, vector diagrams provide clarity on the resultant force or the overall effect of multiple vectors acting simultaneously.

In contrast, a force diagram primarily focuses on illustrating forces acting on a body, often simplifying complex interactions to show equilibrium or net force, rather than representing a broader range of vector types. Space diagrams, while they may depict positions in space, do not specifically characterize vectors or their relationships. Lastly, a magnitude diagram, although it may convey the size of quantities, lacks the specific directional component that is a hallmark of vector representation. Therefore, the best term for a diagram showcasing a system of vectors is indeed a vector diagram.