The sentence which best identifies the main idea of paragraph two is the following one:
B. Scientists are confused by the different ways the rocks move, and the theories involving wind, ice, and mud have not been proven.
Right in the beginning of paragraph two, it is said that scientists are baffled by the variety of rock movement. Then, there is the description of their movement and some possible reasons for it to happen (wind, ice and mud) are suggested by scientists. The paragraph ends with the statement that none of the theories have been confirmed by research. Therefore, letter B summarizes the main idea of the paragraph very well.