D.The natural imagery is developed throughout to indicate that nature continues long after humans do. This is the correct answer.
The first stanza describes the movements of nature. The sea movements ; the birds' songs at sunset indicate the constant cycle of nature. This is compared to the human movement through a traveller who walks to town without noticing the passage of time. The constant cycle of nature represents the endless passage of time. In the second stanza, there is the same comparison. The city is organised by human time. Men go to sleep in their houses at night, however, nature goes on moving as if it were indifferent to this human organization of time. The sea goes on moving. In the third stanza, the comparison is made bewteen ,during day light , the activity on a farm and , again, the natural cycle of natural time. That the traveller will not come back to the shore means that he will never be able to recover time.
