Final answer:
The voltage drop in batteries as they discharge can be explained by the Nernst equation, which shows the d) voltage drop results from changing ion concentrations that affect the cell potential.
Step-by-step explanation:
The Nernst equation is used to describe how the potential of an electrochemical cell, such as a Zn/Cu cell, varies with the concentration of the reacting ions as a battery discharges. As the battery discharges, the concentrations of ions change, leading to a change in the reaction quotient Q in the Nernst equation, which in turn results in a decrease in the cell potential or voltage drop.
This is because the equation accounts for the concentration of reactants and products, which shift as a battery is used. Hence, the correct answer to the question is 'd) Voltage drop is a result of changing ion concentrations'.
Batteries have a limited supply of reactants and, as they discharge, the concentrations of the reactants decrease while the concentration of the products increase.
The Nernst equation demonstrates that when the concentration of reactants is decreased and/or the concentration of products is increased, the potential of the cell drops, which is observed as the voltage drop of a discharging battery.
Moreover, temperature also impacts the voltage as indicated by the Nernst equation, although the temperature effect is not the primary cause of voltage drop during discharge.