**Core Concept**
The hormone responsible for stimulating milk secretion, also known as galactopoiesis, is a crucial component of the lactation process in mammals. This hormone plays a key role in the development and maintenance of the mammary gland during pregnancy and lactation.

**Why the Correct Answer is Right**
The hormone oxytocin is primarily responsible for stimulating milk secretion and ejection from the mammary gland. Oxytocin acts on the myoepithelial cells surrounding the alveoli of the mammary gland, causing contraction and milk ejection. This hormone is released from the posterior pituitary gland in response to suckling or other stimuli, triggering the letdown reflex.

**Why Each Wrong Option is Incorrect**
**Option A:** Prolactin is a hormone involved in milk production (galactopoiesis), but it is not responsible for milk ejection. Prolactin stimulates the growth and differentiation of the mammary gland during pregnancy.
**Option B:** Estrogen and progesterone are essential for mammary gland development and maintenance, but they are not directly involved in milk secretion.
**Option C:** Adrenaline (epinephrine) can inhibit milk ejection, whereas oxytocin stimulates it. Adrenaline is released in response to stress or other stimuli, which can suppress lactation.

**Clinical Pearl / High-Yield Fact**
Oxytocin is often used in medical settings to induce labor or to stimulate uterine contractions. However, its use in lactating women can suppress milk ejection and lead to decreased milk production.

**Correct Answer: C. Oxytocin**