In this case, as with any gradient, the ions will move from high concentration to lower concentration, an aspect that is regulated in cells and used to produce energy, So with this in mind we can proceed to analyze each available option.
a. ions move across a membrane. This must be approached carefully as the movement of ions in a membrane can generate an electrochemical gradient, nonetheless not always the movement of ions will generate a gradient, it all depends on the concentrations of the ions on each side of the membrane.
b. ion concentration changes on one side of a membrane. The difference in ion concentration in a membrane generates a potential differential and an electrochemical gradient, therefore this is the correct option.
c. energy is released. This is not correct as not all electrochemical gradients release energy some others are used to form energy.
d. salts are produced. This is not correct as gradients do not involve the production of a substance in particular.
e. ATP →ADP + Pi. This could be partially correct as many ATPases use ATP to move ions across the membrane nonetheless not all gradients use ATP others generate it.