In the poem the speaker says, "The Carriage held but just Ourselves –
" At the very beginning of the poem the speaker personifies Death. When she says, "Because I could not stop for Death - He kindly stopped for me". This idea that Death stops for the speaker, personifies Death and makes him a character in the poem. This means that "Ourselves" refers to the speaker and Death.
Some people have argued that since Death is personified, so is Immortality and therefore Immortality is also riding in the Carriage with the speaker and Death. However, this is not a widely accepted interpretation.
This line is not talking about his morals, or interests in general. So, from the given answers C and D are off the list from the possible answer. Now, because it says " not unacquainted", it is leaning toward the fact that he had basic or any kind of knowledge beforehand. So, the right answer indeed is A.