I believe the answer is: B. To show that Grendel cannot rely on nature for protection
In the story, Grendel perceive that he is blessed by protection from the nature which would always help him in all of his troubles. But when he somehow got his foot stucked in a tree in front of a bull and when he cried and scream for help from the nature, he did not receive any answer.
A paragraph is a series of sentences that are organized and coherent, and are all related to a single topic. Almost every piece of writing you do that is longer than a few sentences should be organized into paragraphs. This is because paragraphs show a reader where the subdivisions of an essay begin and end, and thus help the reader see the organization of the essay and grasp its main points.