Which of the sentence below best describes one of the main themes in this poem? a. time passes slowly and waits for nobody. b. life is beautiful, and each day brings joy. c. nature is unpredictable, powerful, and threatening.

I'd say that the sentence that best describes one of the main themes in this poem is B. Life is beautiful, and each day brings joy.
Although the narrator wishes winter to be over already, at the end of the poem, he realizes that winter is also beautiful, as well as spring and other seasons may be.