How many potassium ions pass through if the ion channel opens for 1.0 ms?

1.8E-15C/1.6E-19=11,250

Explanation:

The biochemistry that takes place inside cells depends on various elements, such as sodium, potassium, and calcium, that are dissolved in water as ions. These ions enter cells through narrow pores in the cell membrane known as ion channels. Each ion channel, which is formed from a specialized protein molecule, is selective for one type of ion. Measurements with microelectrodes have shown that a 0.30-nm-diameter potassium ion ({\rm{{\rm K}}}^{\rm{ + }} ) channel carries a current of 1.8 {\rm pA}.

it's a b and

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.