Final answer:
Designers make C. conceptual sketches when practicing their skills to capture design ideas and explore possibilities before creating more detailed drawings.
Step-by-step explanation:
The type of sketch that designers make when practicing their skills is conceptual. Conceptual sketches are quick, rough drawings that capture design ideas and explore different possibilities. They are used to communicate and visualize the initial concepts before moving on to more detailed and technical drawings.
For example, a designer may create a conceptual sketch to outline the composition and elements of a set design for a theater production. Thus, when practicing their skills, designers primarily make conceptual sketches to explore and communicate their ideas. Analytical sketches are used to analyze existing designs, observational sketches help improve observation skills, and balloon sketches are a quick and minimalistic way to capture ideas.