An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism?
na+ ions diffusing across the membrane through leakage channels the generation of local currents
k+ ions diffusing through voltage-gated channels neurotransmitters causing chemically gated channels to open

Na+ ions diffusing across the membrane through leakage channels the generation of local currents

Explanation:

An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. There is a voltage difference between the exterior and interior of the cell, called the membrane potential, due the Na+/K+ ATPase pump.  

Starting from the resting potential, the action potential starts when the thershold is reached when Na+ ions difusse across the membrane through channels.Then, the interior of the cell becomes positive and the exterior negative.

The K+ channels take place when the repolarization phase stars, letting K+ ions difusse from the inside of the cells to the exterior.  

"the periodic table is arranged in order of atomic number."