The correct option is
B mankind
“Mankind” means
human beings considered collectively; the human race, which is correct to use here because loss of water is a warning to the people living there, hence Option C is the correct answer. Option A is incorrect because “everything” means every single object or person, which involves living and non living things, which is incorrect here as it cannot be a warning for the non-living things.
Option B is incorrect because “a big warning for someone” means it specifically mentions that one particular person, might be affected which is incorrect because it is for the entire population.
Option D is incorrect because the collection of human population included both men and women, hence incorrect.