Final answer:
The word 'onset' in Emily Dickinson's poem refers to the 'arrival' of a significant moment or event, which is the personified death appearing as the 'King' in the room.
Step-by-step explanation:
In the excerpt from Emily Dickinson's poem "Dying," the meaning of the word onset is most closely related to 'arrival'. The context suggests that it is about a final moment or event, which in this case, is death personified as the 'King' arriving in the room. It does not so much denote the beginning of an action or process (which would be choice B) but rather the coming of a significant event or moment in the narrative of the poem. Therefore, while 'beginning' is somewhat related, 'arrival' captures the essence of the onset in the context of this poem more precisely, reflecting the preparations for the climactic appearance of death.