What makes up about 50 percent of all land areas in United States?

1. Plateaus

2. Plains

3. Mountains

4. Volcanoes

I think 4 is out, but I am not sure about 1,2 & 3

The correct answer is - 2. Plains.

Around 50% of the territory of the USA is occupied by plains. The plains are dominant landforms in the central part of the country, stretching all the way from the Caribbean coast in the south to Canada on the north, and also in big portions of the eastern part of the country, and the south where they are very low plains. The plains in the central part are semi arid prairies by nature but nowadays they are dominated by farmlands, while the ones on the eastern part are covered with either forests or tall green grasses and bushes, and in the south they are marshey territory.

You're right that volcanoes are out. We can find each of the abovementioned landforms in the USA, but about 50% of them are plains. They're specific for central USA and southern coast.

Answer: 2