living in the western part of the country has its problem,--obtaining fresh watewris not the leasta,with which,b,for whichc,of whichd,of that

living in the western part of the country has its problem,--obtaining fresh watewris not the least
a,with which,
b,for which
c,of which
d,of that

原句应该是Living in the western part of the country has its problems,_______ obtaining fresh water is not the least.
前面的 its problem应该是 复数 its problems
所以答案是 c,of which引导非限制性定从并在定从中 作状语 表示范围
也就是 of the problems

Living in the western part of the country has its problem，（）obtaining fresh water is not the least.
a,with which,
b,for which
c,of which
d,of that
C
活在国家的西部有其问题,获得淡水就是其中重要的一个。