Option B is correct: it gives apple trees the quelities of grazing animals to emphasize that the wall is unnecessary.
In the excerpt, the narrator says "[my] apple trees will never get across and eat the cones under his pines". In these two lines, the narrator compares his apple trees to animals which feed on grass and plants and states that his apple trees won't destroy or "feed" on his plants. As a result, according to the narrator, building a fence is unnecessary.