Which of the following means to add explanatory notes to a document or illustration?

The term that means to add explanatory notes to a document or illustration is "annotate." Annotation involves providing additional information, comments, or explanations on a text or visual element, which enhances understanding and provides context. This practice is essential in engineering design, as illustrations often require further clarification to communicate ideas effectively.

For instance, in engineering drawings or sketches, annotations can specify dimensions, materials, or other critical aspects that help viewers understand the design intent. By annotating, you make the document more informative and accessible, ensuring that all stakeholders grasp the nuances of the design.

Other terms like "illustrate," "clarify," and "highlight" relate to visual communication in some capacity but do not specifically refer to the act of adding explanatory notes. Illustrate focuses on creating visual representations, clarify is more about making something less confusing, and highlight means to draw attention to particular aspects within a document without necessarily adding explanatory content.