The concentrations of ions are very different inside and outside a nerve cell due to .

Sodium-Potassium Pumps

Explanation:

Sodium-Potassium Pumps written as Na-K Pumps are ion channels found in the cell membrane of nerve cells. These pumps are responsible for maintaining the different ion concentrations across the membrane and help in polarization and depolarization of the membrane by influencing membrane potential. During ion exchange these pumps transfer three sodium ions from inside to outside of the cell and two potassium ions from outside to inside of the cell. In this way they maintain the ion concentration gradient across membrane.

a polar bond is formed when electrons are shared unevenly between atoms

the answer is d.) potassium