Living in the central Australian desert has its problems,________obtaining water is not the least.
A.for which B.to which C.of which D,in which

鉴于每次按顺序编号后就找不到题目的出处，所以现在直接用页码题号编号，方便日后知道答案后重新写到书上。

楼上两位都做错了 答案应该选C）of which

此题考核的是定语从句“介词+关系代词”结构中介词的选择。
此题介词的选择主要需要依靠表示形容词最高级范围时所用的介词。因为题目中出现了least，给予了暗示。
不如举两个例句说明：
He is the tallest (student) of all (the students).
He is the tallest (student) in his class.
回到题目，题干中出现了problems，也即为先行词，相当于第一个例句中students的位置。通过类比，可知应该选C。