What type of plant did the Navajo soldiers rely on for hydration training in the desert?

The prickly pear cactus is the correct answer because it is a specific type of cactus that not only grows in arid environments but also provides a source of hydration for those in the desert. This plant has pads that contain moisture, making it a valuable resource for the Navajo soldiers during their training in the desert, where water is scarce. They learned to identify and utilize this cactus for survival, demonstrating their knowledge of the local flora.

The other choices, while related to plants that might thrive in dry conditions, are either too broad or not as directly connected to the context of hydration training. For example, "cacti" covers a wide array of species but does not specify the practical aspects of hydration provided by prickly pear. "Xerophyte" refers broadly to plants adapted to dry environments, and "suculent" encompasses various water-retaining plants, again lacking the specificity of how prickly pear cactus was utilized by Navajo soldiers. Thus, choosing prickly pear cactus highlights both the specific plant used and its practical benefits for hydration in a desert training scenario.