[A]: In vertebrates, motor fibres to striped muscle, parasympathetic fibres to smooth muscle, and preganglionic sympathetic fibres are cholinergic.
[R]: Cholinergic designates the type of nerve fibre that releases acetylcholine from its ending when stimulated by a nerve impulse

Options

A. (a) If both A and R are true and R is the correct explanation of A
B. (b) If both A and R are true but R is not the correct explanation of A
C. (c) If A is true and R is false
D. (d) If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that in vertebrates, motor fibers to striped muscles, parasympathetic fibers to smooth muscles, and preganglionic sympathetic fibers are cholinergic. This means that these fibers release acetylcholine when stimulated.

2. **Analyzing Motor Fibers**:
- Motor fibers are responsible for stimulating muscle contraction and relaxation.
- In striped (skeletal) muscles, these motor fibers are indeed cholinergic, meaning they release acetylcholine, which is essential for muscle activation.

3. **Examining Parasympathetic Fibers**:
- Parasympathetic fibers are part of the autonomic nervous system and control involuntary functions.
- These fibers, particularly those in the vagus nerve, also release acetylcholine to regulate smooth muscle movements in organs such as the heart and lungs.

4. **Looking at Preganglionic Sympathetic Fibers**:
- Preganglionic sympathetic fibers are involved in the fight-or-flight response.
- These fibers are also cholinergic, releasing acetylcholine to prepare the body for stressful situations.

5. **Understanding the Reason**:
- The reason states that cholinergic fibers are those that release acetylcholine when stimulated.
- This definition aligns perfectly with the assertion, as all the mentioned fibers (motor, parasympathetic, and preganglionic sympathetic) are indeed cholinergic.

6. **Conclusion**:
- Both the assertion and reason are true.
- The reason provides a correct explanation for the assertion.
- Therefore, the correct answer is option A: both assertion and reason are true, and the reason is the correct explanation of the assertion.