Final answer:
The Missouri River was the river most commonly followed by trails such as the Oregon Trail to ensure access to water for explorers and settlers travelling west. It was vital for survival and agriculture and was also explored for potential commerce routes.
Step-by-step explanation:
The trail mentioned in the question likely refers to the historically significant trails used during the westward expansion of the United States, such as the Oregon Trail or the Mormon Trail. These trails followed rivers to provide a reliable source of water in an arid landscape. The river that was generally followed by these trails to provide access to water was the Missouri River (d). Explorers and settlers would have relied on the river for drinking, cooking, and agricultural purposes, making it an essential feature of their journey westward.
The Missouri River, being the longest river in North America, played a crucial role in the exploration and expansion to the west. It was the principal stream that the explorers of the early 19th century, like Lewis and Clark, followed in searching for a water communication route that could facilitate commerce across the continent, including trade with the Pacific regions. The importance of water-sourced routes is underscored by the fact that, during those times, riverways were the most practical and direct means for both transportation and supply of essentials such as water.
Other rivers mentioned, like the Colorado River and the Rio Grande River, have their own historical and geographical significance. For instance, the Colorado River is known for the Hoover Dam and its role in providing water to a vast area and several states. However, regarding the trails specifically for westward expansion, they predominantly followed the course of the Missouri River.