The correct answer is hCG.
The corpus luteum is a hormone-producing structure in female ovaries which is responsible for establishing and maintaining pregnancy in females. It is formed in every menstrual cycle and one of its important responsibilities is to secrete progesterone. If fertilization occurs, the hormone human chorionic gonadotropin (hCG) is released in order to signal the corpus luteum to keep secreting progesterone and create the necessary conditions for the zygote to develop.