**Core Concept:**
The question is asking about a substance or process that stimulates the production of milk proteins, specifically in the context of breastfeeding. In lactation, milk synthesis occurs primarily in breast tissue, particularly in alveoli, due to hormonal and local factors.

**Why the Correct Answer is Right:**
The correct answer, **Option D: Prolactin**, is the hormone primarily responsible for stimulating milk synthesis in lactating breasts. Prolactin is secreted by the anterior pituitary gland in response to various stimuli, including breast stimulation, suckling, and hypothalamic signals. It promotes alveolar cell growth, milk-producing cell proliferation, and milk ejection reflex (also known as the let-down reflex).

**Why Each Wrong Option is Incorrect:**

A. **Option A: Oxytocin (Oxytocin receptor):** Oxytocin plays a role in the milk ejection reflex, but it is not directly involved in stimulating milk protein synthesis.

B. **Option B: Glucocorticoids:** Glucocorticoids are primarily involved in regulating glucose homeostasis and immune responses, not milk protein synthesis.

C. **Option C: Thyroid hormones:** Thyroid hormones are essential for fetal development and overall growth, but they are not directly involved in stimulating milk protein synthesis.

**Clinical Pearl / High-Yield Fact:**
Understanding the role of prolactin in lactation is crucial for healthcare professionals, especially in cases where lactation is impaired, such as in postpartum women with low prolactin levels or those receiving dopaminergic medications that block prolactin release. Monitoring prolactin levels can help diagnose and manage these conditions, ensuring proper breastfeeding and infant growth and development.