**Core Concept**
Relaxin is a hormone that plays a crucial role in pregnancy by relaxing the pelvic muscles and ligaments, facilitating childbirth. It is also involved in the regulation of fetal development and maternal physiology.

**Why the Correct Answer is Right**
During pregnancy, relaxin is secreted by the corpus luteum and the placenta. The hormone binds to its receptor, relaxin receptor 2, which is present in various tissues, including the uterus, vagina, and pelvic ligaments. This binding causes the relaxation of these tissues, allowing for the necessary changes in the pelvic cavity during childbirth. Relaxin also has a role in the regulation of fetal development, particularly in the formation of the fetal skeleton.

**Why Each Wrong Option is Incorrect**
**Option A:** The incorrect options for relaxin secretion during pregnancy are:
* Oxytocin: Oxytocin is a hormone involved in uterine contractions and milk letdown, but it is not responsible for relaxin secretion.
* Progesterone: Progesterone is a hormone essential for maintaining pregnancy, but it is not the primary source of relaxin.
* Estrogen: Estrogen is a hormone involved in fetal development and maternal physiology, but it is not directly responsible for relaxin secretion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that relaxin levels are elevated during pregnancy, which can lead to an increased risk of ligamentous laxity and joint instability. This is particularly important for clinicians managing patients with pre-existing joint conditions or those at risk of osteoporosis.

**Correct Answer:** C. The placenta.