Final answer:
The Coconino Sandstone is composed of cross bedded dune deposits, characterized by large-scale cross-bedding and well-rounded sand grains, indicative of ancient sand dunes. The correct option is c.
Step-by-step explanation:
The rock unit composed of cross bedded dune deposits is the Coconino Sandstone. This geological formation consists of sandstone characterized by its large-scale cross-bedding and very well-rounded sand grains. The Coconino Sandstone is notable for its extensive cross beds, indicating ancient sand dunes similar to those that can be found in deserts today.
Examples of cross-bedding can be observed in outcrops such as those in Coyote Gulch, Utah, and the iconic landscapes such as the Garden of the Gods in Colorado, where sedimentary rocks have been tilted and eroded over time.
Hence, Option c is correct.