**Core Concept**
The question tests the understanding of **hypothalamic-pituitary axis** and the management of **hypopituitarism** following neurosurgery. The **pituitary gland** plays a crucial role in regulating various hormonal functions in the body.

**Why the Correct Answer is Right**
The correct answer is **Cortisol**, as it is essential for the body's response to stress, regulating blood sugar, and aiding in metabolism. In the context of hypopituitarism, the lack of **Adrenocorticotropic hormone (ACTH)** leads to decreased cortisol production. Replacing cortisol is critical to prevent **adrenal crisis**, a life-threatening condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Thyroid hormone replacement is important but not as immediate as cortisol, as the body has a longer threshold for thyroid hormone deficiency.
**Option B:** Growth hormone replacement, although crucial for growth and development in children, is not as urgent as cortisol replacement.
**Option D:** Insulin replacement is not directly related to pituitary function but to pancreatic function.

**Clinical Pearl / High-Yield Fact**
In post-surgical hypopituitarism, **cortisol replacement** should be initiated promptly to prevent adrenal crisis, highlighting the importance of understanding the hierarchy of hormone replacement in such scenarios.

**Correct Answer:** C. Cortisol.