[A] : Synaptic vesicles fuse with membrane and release neurotransmitter molecules.
[R]: Neurotransmitter attaches to receptor site and open sodium channels to allow sodium to flow into post synaptic cell causing depolarisation.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution

1. **Understanding the Assertion**:
- The assertion states that "synaptic vesicles fuse with the membrane and release neurotransmitter molecules." This is a fundamental process in synaptic transmission where neurotransmitters are released into the synaptic cleft.

2. **Understanding the Reason**:
- The reason provided is that "neurotransmitter attaches to receptor site and opens sodium channels to allow sodium to flow into the post-synaptic cell causing depolarization." This describes the effect of neurotransmitters on the post-synaptic neuron, leading to depolarization.

3. **Analyzing the Process**:
- When a nerve impulse reaches the synaptic knob, calcium ions enter the synaptic knob. This influx of calcium ions triggers the synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
- The released neurotransmitters then bind to specific receptors on the post-synaptic neuron. This binding opens ligand-gated sodium channels, allowing sodium ions to flow into the neuron.

4. **Result of the Process**:
- The influx of sodium ions reduces the negative charge inside the post-synaptic neuron, leading to depolarization. This depolarization can generate an action potential if the threshold is reached.

5. **Conclusion**:
- Both the assertion and reason are true statements regarding synaptic transmission. However, the reason does not directly explain the assertion; rather, they describe different aspects of the synaptic process. Therefore, the correct conclusion is that both statements are true, but the reason is not the explanation for the assertion.

### Final Answer
Both the assertion and reason are true, but the reason is not the explanation for the assertion.