The horse (Equus ferus caballus) has the maximum number of chromosomes among the given options, with 64 chromosomes (32 pairs). In comparison, Drosophila (fruit fly) has only 8 chromosomes, and humans have 46 chromosomes (23 pairs). Ascaris, a type of roundworm, has 2 chromosomes in each cell. Therefore, the organism with the maximum number of chromosomes in this list is the horse.