Presenters at the seminar, one who is blind, will demostrate adaptive equipment that allows visually inmpaired people to use computers.

a. one who
b. one of them who
c. and one of them who
d. one of whom
e. one of which

when is one to use "whom" and when to use "who"?
would choice b be correct it "whom" was used and there was no choice d?
Is the word "whom" the only reason why choice d IS CORRECT?