The answer would be, "D". Nice loli picture.
Animals cells are found inside the animals body. We have no exact place where it is found ;)
Genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).
Carbon dioxide because every other option requires an input.
When the body becomes dehydrated, a chemical signal is sent that causes nephrons in the kidneys to absorb more water and excrete more salts. This is an example of a <em>D Feedback loop</em>.