**Core Concept**
Prolactin secretion is regulated by a complex interplay of neurotransmitters and hormones. The primary inhibitory influence on prolactin secretion is exerted by dopamine, which acts through the D2 receptor to decrease prolactin release.

**Why the Correct Answer is Right**
Dopamine is the primary inhibitor of prolactin secretion, and its action is mediated through the D2 receptor. When dopamine binds to the D2 receptor, it activates a G-protein coupled pathway that inhibits the production and release of prolactin from the lactotroph cells in the anterior pituitary gland. This inhibitory effect is crucial in maintaining normal prolactin levels, as excessive prolactin secretion can lead to various symptoms and disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because dopamine is the primary inhibitor of prolactin secretion, not the primary stimulator.
**Option B:** This option is incorrect because dopamine does not stimulate prolactin secretion; instead, it inhibits it.
**Option C:** This option is incorrect because while dopamine is a neurotransmitter that can stimulate other physiological processes, its primary role in the regulation of prolactin secretion is as an inhibitor.

**Clinical Pearl / High-Yield Fact**
The inhibition of prolactin secretion by dopamine is a critical aspect of maintaining normal prolactin levels. Dopamine agonists, such as bromocriptine, can be used to treat conditions characterized by hyperprolactinemia, such as prolactinomas.

**Correct Answer: D. Dopamine.**