I would go with B. 4000 is the number of drive-in (theatres). It can't be amount.
as many what??? B is an incomplete sentence.
Fewer than ... (countable noun) that amount...
Agreed, it should be as many as X.
I would have liked D the number is less than quarter of that amount. But the choice D is missing "of" so it looks awkward
Hence C is the best one