It is well known that rice can be only grown in areas ( ) en

C.
where 引导 areas 的定语从句：有充足水分的地区.
where (以及when,why)引导定语从句时,从句中还是应该有完整的主谓结构.
This is the restaurant where they had dinner last night.
（在定语从句 where they had dinner last night 中,they 是主语,had 是谓语动词.）
本题中 ...where there is enough water,定语从句是 there be ...句型,结构也是完整的.
至于 D 项,首先是单复数的问题.既然 areas 是复数,就应该是 have.其次,个人认为 which have 也不如 where there is 好.“那些地区有充足的水.”我们是说 There is enough water in those areas.而不习惯说 Those areas have enough water.

D.
which has enough water 是一个定语从句.先行词是areas,which在定语从句中作主语.where只能在定语从句中做状语.

C:where there is
地点状语从句 用where 引导

D
地点状语从句