The correct option is
C dry
Most of the wells have gone dry, means that with time, eventually, the wells have lost their water, hence option B is the correct answer.
Option A is incorrect because “well have gone down” means that the wells have lowered, which makes no sense unless the water levels were specified, hence incorrect.
Option C is incorrect because “most wells have gone up” means they have increased in number which is incorrect, because the water level is being mentioned here.
Option D is incorrect because “empty” means absence of any substance, which is incorrect because the wells are not necessarily empty, they have just lost their water levels which is important to mention, they might still have other small substances in the well.