**Core Concept**
The onset of labor is a complex process involving the coordination of hormonal, mechanical, and neural signals. It is influenced by the maturation of the fetus, changes in the uterine environment, and the relaxation of the pelvic floor muscles.

**Why the Correct Answer is Right**
The correct answer is likely to be related to the role of prostaglandins in triggering labor. Prostaglandins, particularly PGE2, stimulate uterine contractions by binding to specific receptors on the uterine smooth muscle cells. This binding causes an increase in intracellular calcium, leading to muscle contraction. The production of prostaglandins is increased in the uterus during labor, and they are thought to play a key role in initiating and maintaining contractions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because oxytocin is a hormone that stimulates uterine contractions, but it is not typically responsible for triggering the onset of labor. Instead, it is often used to augment or induce labor in a medical setting.
**Option B:** This option is incorrect because estrogen is a hormone that supports the growth and development of the fetus, but it is not directly involved in triggering labor.
**Option C:** This option is incorrect because relaxin is a hormone that relaxes the pelvic floor muscles and is involved in the process of labor, but it is not the primary trigger for the onset of labor.

**Clinical Pearl / High-Yield Fact**
The onset of labor is often triggered by the maturation of the fetal lungs, which produce surfactant and stimulate the production of prostaglandins in the uterus.

**Correct Answer: C. Prostaglandins. Prostaglandins are involved in triggering the onset of labor.**