The correct answer is option 4, XY. In humans, sex is determined by the presence of either two X chromosomes (in females) or one X and one Y chromosome (in males). The Y chromosome carries the genes responsible for the development of male characteristics, such as testosterone production and the development of male reproductive structures. The presence of the Y chromosome triggers the development of testes in the fetus, leading to the production of androgens, which are responsible for the development of male secondary sexual characteristics. Therefore, it is the presence of the Y chromosome that is responsible for manliness in males. The other options listed in the question are incorrect.