Because I could not stop for Death — He kindly stopped for me —
The Carriage held but just Ourselves —
And Immortality.

We slowly drove — He knew no haste
And I had put away
My labor and my leisure too,
For His Civility —

We passed the School, where Children strove
At Recess — in the Ring —
We passed the Fields of Gazing Grain —
We passed the Setting Sun —

Or rather — He passed Us —
The Dews drew quivering and chill —
For only Gossamer, my Gown —
My Tippet — only Tulle —

We paused before a House that seemed
A Swelling of the Ground —
The Roof was scarcely visible —
The Cornice — in the Ground —

Since then — ’tis Centuries — and yet
Feels shorter than the Day
I first surmised the Horses’ Heads
Were toward Eternity —
what is the central topic of the poem

The central topic of the poem is the immortality of one's life/ soul from a mortal's standpoint, the question of what happens after one dies and what is perceived of death.

Explanation:

The central topic of any literary work is the central theme or point that the writer/ author talks about in the text. This means that the central topic is the central or main theme that the author discusses and wants to share with the readers.

Emily Dickinson's poem "Because I Could Not Stop For Death" talks about life and death. The speaker 'narrates' how she was visited by "Death" and taken to her grave on a "carriage". The main topic or idea of the whole poem is that of man's immortality, the interpretation of death and life after death. The speaker delves into the interpretation of immortality from the standpoint of a mortal.  

i think the answer is b