My reasons :-
When we deal with countable numbers, use fewer. Less is used an uncountable quantity eg. less water.
Therefore this leaves us with B and C
C is wrong, because of the word "amount". Amount as Ritesh rightly pointed out is an uncountable quantity. eg. amount of water, amount of good will.. etc. Therefore, fewer and amount won't go together.
I used exactly the same POE strategy.
amount - uncountable item
fewer - countable
less - uncountable
The OA is indeed B.