What is one of the primary reasons for the Grand Canyon's popularity as a tourist destination?

The Grand Canyon's popularity as a tourist destination is primarily attributed to its unique geological features and vast size. It is renowned for its stunning and intricate rock formations that have been sculpted over millions of years by the Colorado River and geological processes. Visitors are captivated by the sheer grandeur of its scale—stretching over 277 miles in length, up to 18 miles in width, and reaching depths of over a mile. This immense natural wonder not only provides breathtaking vistas but also offers a glimpse into Earth's geological history, making it a fascinating site for both casual visitors and serious geologists.

While the other options mention aspects that contribute to the visitor experience, such as hiking trails or proximity to cities, they do not capture the fundamental reason for the canyon’s draw. The unique geological features and sheer size are what truly distinguish the Grand Canyon and entice millions of people each year. These characteristics create opportunities for photography, exploration, and education about natural history, further solidifying its status as a beloved landmark.