C. The awareness of impending death came about slowly, subtly
Explanation:
According to the given excerpt, the comment from Dickinson about expecting death in the line “since then — tis centuries — and yet/ feels shorter that the day/ I first surmised the horses head/ were toward eternity" is the awareness of impending death came about slowly, subtly.
This is because he slowly realizes as he feels shorter that day and finding out that the horses egad was toward eternity.
