**Core Concept**
Postganglionic sympathetic fibres are a crucial component of the autonomic nervous system, responsible for transmitting signals from the sympathetic nervous system to various effector organs. These fibres play a significant role in regulating various physiological processes, including the 'fight or flight' response.

**Why the Correct Answer is Right**
Postganglionic sympathetic fibres are unmyelinated, preganglionic fibres that originate from the sympathetic chain or the superior cervical ganglion. These fibres release the neurotransmitter acetylcholine, which stimulates the postganglionic neuron to release the neurotransmitter norepinephrine. The postganglionic fibres are unmyelinated, allowing for a slow, sustained release of norepinephrine, which is essential for the regulation of various autonomic functions.

**Why Each Wrong Option is Incorrect**
**Option A:** **A alpha** fibres are myelinated, preganglionic fibres that originate from the central nervous system and are involved in the parasympathetic nervous system, not the sympathetic nervous system.
**Option C:** **B fibres** are myelinated, preganglionic fibres that originate from the central nervous system and are involved in the parasympathetic nervous system, not the sympathetic nervous system.
**Option D:** **A gamma** fibres are not a specific type of fibre associated with the autonomic nervous system.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that postganglionic sympathetic fibres are unmyelinated and release norepinephrine, which is a key neurotransmitter involved in the regulation of various autonomic functions, including blood pressure, heart rate, and respiratory rate.

**Correct Answer:**
✓ Correct Answer: B. C