This one had two possible answers, but the most correct answer is …
C. one square foot for every 1,500 square feet of floor
The crawl space is unconditioned, so ventilation is required. The required crawl space ventilation is one square foot for every 150 square feet of floor (or one square inch for every square foot of floor), unless there is a vapor retarder (as shown in the diagram) in which case the required ventilation is a square foot for every 1,500 square feet of floor (or one square inch for every 10 square feet of floor).