**Core Concept**
The milk ejection reflex, also known as the letdown reflex, is a physiological process that enables lactation in breastfeeding women. This process involves the coordinated action of several hormones to facilitate milk release from the mammary glands.

**Why the Correct Answer is Right**
Milk ejection is primarily mediated by the hormone oxytocin, which is released by the posterior pituitary gland in response to suckling or other stimuli. Oxytocin acts on the myoepithelial cells surrounding the alveoli of the mammary gland, causing them to contract and release milk into the ducts. This process is essential for effective breastfeeding and milk supply.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because prolactin, while crucial for milk production, is not directly involved in the milk ejection reflex.
**Option B:** This option is incorrect because vasopressin, also released by the posterior pituitary gland, has a different primary function, regulating water reabsorption in the kidneys.
**Option C:** This option is incorrect because growth hormone, produced by the anterior pituitary gland, stimulates growth and development but is not directly involved in milk ejection.
**Option D:** This option is incorrect because insulin, a hormone produced by the pancreas, regulates blood glucose levels but is not involved in the milk ejection reflex.

**Clinical Pearl / High-Yield Fact**
Oxytocin's role in milk ejection is essential for effective breastfeeding, and any disruption in this process can lead to inadequate milk supply or other lactation issues.

**Correct Answer: A. Oxytocin**