OA is C
can someone explain the diff between B & C
(B) Problems of managing water and resources
>> "problems of managing water" sounds as if managing water itself is a problem, also sounds awkward.
(C) Problems in the management of water and other resources
>> This more clear, "problems that are faced in the management of the water". This is idiomatic as razlulz pointed out.
One more point: "other resources" actually makes it clear that "resources" are something other than water.
You can frame sentence (C) with & without "other" & read
loudly, check which one your ears like