C because it's proper English.
Answer:
claim
Explanation:
A main claim is a position that the author makes in relation to an argument previously presented, but not only that, since this position governs all the ideas and arguments that the author will give throughout the text.
The text shown above shows a position, that is, an opinion of an author about something that is being done in the educational field and that is not being beneficial to anyone. As we saw before, a main claim is a position, so we can say that main claim is the answer to your question.
The answer would be A.
The description of "Unlike normal rocks, they have trails etched behind them as if they have traveled across the sand.", provides detail to support the main idea that "drifting" rocks are strange. Some keywords that will help you understand the main idea that "Drifting Rocks are Stange" :
°Unlike
°Traveled
°Trails