Questions: Increased calcium ion permeability of the presynaptic terminal cell membrane is caused by.... a) an action potential. b) a decrease in the concentration of sodium ions. c) acetylcholine. d) carbon dioxide.

The answer is the first one (a): an action potential.

Explanation for each option: a) An action potential: Correct. An action potential arriving at the presynaptic terminal causes voltage-gated calcium channels to open, increasing the permeability of the presynaptic terminal cell membrane to calcium ions. b) A decrease in the concentration of sodium ions: Incorrect. A decrease in sodium ion concentration does not directly cause an increase in calcium ion permeability at the presynaptic terminal. c) Acetylcholine: Incorrect. Acetylcholine is a neurotransmitter released into the synaptic cleft, but it does not directly increase calcium ion permeability at the presynaptic terminal. d) Carbon dioxide: Incorrect. Carbon dioxide does not play a role in increasing calcium ion permeability at the presynaptic terminal.

Summary: The increased calcium ion permeability of the presynaptic terminal cell membrane is caused by an action potential.