The Birches is a poem where the author describes the reality of adulthood, “swinging birches” is the escape to that. Before he says “Soon the sun's warmth makes them shed crystal shells” he is explaining and comparing the fact that childhood doesn’t stay long as –ice storms- , and then here comes the description, when the sun arises, the ice cracks off the trees. The other three options cannot be possible as they are completely out of context, the poem does not talk about –glass figurines, rocks or beaches-.
