**Core Concept**
Prolactin is a hormone secreted by the anterior pituitary gland that plays a crucial role in lactation, particularly during the postpartum period. It stimulates the growth and differentiation of mammary glands, prepares them for milk production, and regulates the secretion of milk.

**Why the Correct Answer is Right**
Prolactin's primary function during lactation is to induce milk synthesis and secretion. This is achieved through the stimulation of mammary gland cells, specifically alveolar cells, to produce milk proteins and lipids. Prolactin binds to its receptor, which activates a signaling cascade that ultimately leads to the expression of genes involved in milk production. The hormone also helps to regulate the growth of the mammary gland, ensuring that it is adequately developed to produce milk for the newborn.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while prolactin does have a role in regulating growth and development, it is not its primary function during lactation.

**Option B:** This option is incorrect because oxytocin, not prolactin, is primarily responsible for the letdown reflex, which stimulates milk ejection.

**Option C:** This option is incorrect because while prolactin does have a role in regulating reproductive processes, it is not its primary function during lactation.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that prolactin levels typically drop after weaning, and its deficiency can lead to galactorrhea, a condition characterized by the spontaneous flow of milk from the nipple.

**Correct Answer: B. Milk synthesis and secretion.**