Concept: This question tests your understanding of synaptic transmission, specifically the role of ions in triggering neurotransmitter release.

Why (D) is correct: When an action potential reaches the axon terminal, it depolarizes the membrane, opening voltage-gated calcium ion (Ca${}^{2+}$) channels. The influx of Ca${}^{2+}$ into the presynaptic terminal triggers the fusion of synaptic vesicles (containing neurotransmitters) with the presynaptic membrane, leading to the release of chemical messengers into the synaptic cleft.

Why other options are wrong:

• (A) Mg${}^{2+}$: Magnesium ions are important cofactors for many enzymes but do not directly trigger neurotransmitter release.
• (B) Fe${}^{2+}$: Iron ions are crucial for various cellular processes (e.g., oxygen transport), but not for synaptic vesicle fusion.
• (C) Cl${}^{-}$: Chloride ions are involved in hyperpolarization and inhibitory postsynaptic potentials, not in triggering neurotransmitter release.

Common Mistake: Students might confuse the roles of different ions in neuronal signaling. Remember that Ca${}^{2+}$ is the primary trigger for neurotransmitter exocytosis.

NEET Tip: Think of Ca${}^{2+}$ as the 'key' that unlocks the release of neurotransmitters from the presynaptic terminal.

Correct Answer: (D)