The hormone responsible for placenta formation is:

Explanation

After fertilization and the formation of a zygote, the developing embryo starts producing hCG. This hormone is essential for maintaining the corpus luteum, a structure formed from the ruptured follicle after ovulation. The corpus luteum, in turn, continues to produce progesterone, which helps sustain the early pregnancy. As the embryo implants into the uterus, the trophoblast cells, a layer of cells surrounding the developing embryo, play a crucial role in forming the placenta. These trophoblast cells produce hCG, and it is the rising levels of hCG that prevent the corpus luteum from degenerating. The sustained production of progesterone from the corpus luteum, in collaboration with other hormones, supports the early development of the placenta.