Which of the following hormones is not secreted by the human placenta ?

The correct option is Relaxin.

To understand why this is the correct answer, let's look at the hormones produced during pregnancy and their sources:
The human placenta acts as a temporary endocrine gland during pregnancy and secretes several essential hormones to support fetal development and maintain pregnancy. These include:

1. Human Chorionic Gonadotropin (hCG): Secreted in large amounts by the placenta early in pregnancy to maintain the corpus luteum and stimulate progesterone production.
2. Estrogen and Progesterone: Initially produced by the corpus luteum, the placenta takes over the production of these hormones in large quantities during the later stages of pregnancy to support uterine growth and fetal development.

In contrast, Relaxin is a hormone primarily secreted by the corpus luteum in the ovary (and later by the decidua/uterus), but not by the placenta itself. Relaxin functions to relax the ligaments in the pelvis and soften/widen the cervix in preparation for childbirth.

Therefore, among the given options, Relaxin is the hormone that is not secreted by the human placenta.