**Core Concept**
The 'milk secretion reflex' or galactopoiesis is a physiological process initiated by the suckling stimulus in lactating females, leading to milk ejection and secretion. This process is primarily mediated by the neuroendocrine system and involves the release of specific hormones.

**Why the Correct Answer is Right**
The hormone involved in the 'milk secretion reflex' is oxytocin. Oxytocin is released from the posterior pituitary gland in response to suckling stimuli. It causes contraction of the myoepithelial cells surrounding the alveoli of the mammary gland, resulting in milk ejection. Oxytocin also stimulates the release of prolactin, which is essential for milk production and secretion.

**Why Each Wrong Option is Incorrect**
**Option A:** Prolactin is involved in milk production but not directly in the milk secretion reflex.
**Option B:** Estrogen and progesterone are essential for milk production during pregnancy but not directly involved in the milk secretion reflex.
**Option C:** Adrenaline can suppress milk production, particularly in the early stages of lactation, but it is not directly involved in the milk secretion reflex.

**Clinical Pearl / High-Yield Fact**
Oxytocin is often used clinically to induce milk ejection during breastfeeding difficulties, but it should be used with caution to avoid excessive milk ejection and breast engorgement.

**Correct Answer: C. Oxytocin**