Final answer:
Guillermo del Toro's sketchbook served as a creative idea bank for his films, where he captured and developed ideas for characters, settings, and visual aesthetics.
Step-by-step explanation:
The primary function of Guillermo del Toro's sketchbook was B. It served as a creative idea bank for his films. As a renowned filmmaker, del Toro used his sketchbook to jot down and visualize ideas for characters, settings, and overall visual aesthetics for his films. The sketches served as a starting point for the development of his movies.