**Core Concept**
The question is testing your knowledge of the autonomic nervous system's control over sweat glands. The autonomic nervous system uses two main neurotransmitters to regulate sweat gland secretion: acetylcholine (ACh) and adrenaline (also known as epinephrine). This control is mediated through the sympathetic and parasympathetic divisions of the autonomic nervous system.

**Why the Correct Answer is Right**
The correct answer involves the neurotransmitter released by nerve fibers innervating sweat glands. In the case of eccrine sweat glands, which are responsible for thermoregulation, the primary neurotransmitter is acetylcholine (ACh). ACh stimulates the sweat glands to secrete sweat, which helps to cool the body. The release of ACh is mediated by the parasympathetic nervous system, specifically through the cholinergic transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable, since we don't have information about this option.
**Option B:** Incorrect, because adrenaline is primarily released by the adrenal medulla, not by nerve fibers innervating sweat glands.
**Option C:** Incorrect, because norepinephrine is a neurotransmitter involved in the sympathetic nervous system's control of various functions, including vascular tone and heart rate, but not primarily in sweat gland secretion.

**Clinical Pearl / High-Yield Fact**
The autonomic nervous system's control over sweat glands is an example of the cholinergic and adrenergic transmission, which are fundamental concepts in pharmacology and physiology. Understanding these mechanisms is essential for diagnosing and treating various conditions, such as hyperhidrosis and hypohidrosis.

**Correct Answer:** A. Acetylcholine (ACh).