Read these lines from linda pastan's "erosion": we are slowly undermined. grain by grain. . how does the poem establish a sense of erosion? a. repetition of "grain" b. the word "slowly" c. strategic line breaks d. none of the above

I would say both A and C, the repetition of grain shows the monotony of the action, slowly breaking down. The line break also shows the long winded process of slowly breaking down over time, eroding if you will.
If I was answering this I would say both were correct, as erosion is the slow breaking down over time.