An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism?

The generation of local currents.

An action potential at the axon hillock generates local currents that depolarize nearby sections of axon to threshold. This change in membrane potential causes voltage-gated Na+ channels to open in the adjacent axon segment. This generates/regenerates the action potential in the adjacent segment, causing the action potential to propagate away from the axon hillock.

i think it's c

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!