The correct statement about the Oregon Trail is:
a. It took from four to six months to travel.
The Oregon Trail was a significant route used by settlers moving westward, and the journey typically took several months to complete. The other statements are not accurate regarding the trail: it did not require protection by U.S. troops, it was primarily used by settlers rather than merchants (merchants usually preferred different routes), and it did not run through the Sierra Nevada, as it connected Missouri to the Oregon Territory primarily through what is now the central United States.