Answer: a. the coming of winter.
In these lines, the speaker describes the coming of winter. He states how, as the winter comes, the flocks of sheep go from the fields to the stables, to protect themselves from the weather. The rivers get more violent and the rocks colder. "Philomel" refers to the nightingale, so the line "and philomel becometh dumb" means that the birds have stopped singing. All of these describe how nature changes in winter.
