The correct answer is - b. 50.
The United States have approximately 50 national parks, or to be more specific, there's 58 of them on the territory of this vast North American country. One of the best known and most visited parks that are a global tourist attraction are the Yellowstone National Park, The Grand Canyon National Park, and the Yosemite National Park. There are also many national parks in the United States that we can easily say are underrated and are in the shadow of these three, but at least that is good for the nature because the less people are visiting them, the less damage will be done to them.