He is quite worn out from years of hard work. He is not the man ( ) he was twenty years ago.
A. which B. that C. who D. whom
I chose D, but the answer is B. Who can explain it for me? Thank you.
my first thought is that "whom" is a object case of "who", and since the chap is the subject of this verb to be, then the object (or accusative) case would not be appropriate ...

as for the use of which, that etc - these choices are not always clear to native speakers, there are some rules, but largely ignored! I wouldnt stress too much about this one!
My choice is B 'cause it really fixes that blank without any confusion