well well well !!! computers are idiots .not WE!!!
coming to details of question, this question is being asked in GATE(graduate aptitude test in engineering) in india and there IS an answer to this one which i dunno.
i had worked on subnets problems like given IP address and 'n' systems in network etc etc.....
but this Q is not asked directly.
I think 'D' is correct answer. but i dont have solution!