How is the underlined prepositional phrase functioning in the following sentence?

our dog runs in circles when we come home.

question 6 options:

a)
adjective phrase modifying "dog"

b)
adverb phrase modifying "runs"

c)
adverb phrase modifying "come home"

d)
adverb phrase modifying "when

explanation:

the silver bells of the sleds are merry and keep time in the winter nights while the sky twinkles happily. the golden bells of weddings are delightful in their peaceful happiness, foretelling a rapturous future. meanwhile, the brazen alarm bells scream frightfully in the night, with a discordant and desperate sound. in their clamor, these bells convey terror, horror, and anger. finally, the iron bells are solemn and melancholy, while those in the church steeple are like ghouls who feel happiness. the king of the ghouls, who rings the bells, cheerfully keeps time with the moaning and groaning bells.