Which of the following statements about electrochemical cells is true?

The selected answer is accurate because cell potentials in electrochemical cells can indeed vary based on the concentrations of the reactants and products involved in the half-reactions. The Nernst equation describes this relationship, allowing for the calculation of cell potential under non-standard conditions. According to this equation, the potential of a cell changes with the concentrations of the species involved, reflecting how the driving force for the reaction is influenced by the relative amounts of reactants and products.

In contrast, the other statements do not hold true in the context of electrochemical cells. While some cells can operate under standard conditions, it is not universally true that all electrochemical cells do; many operate at varied conditions depending on specific experimental setups. The claim that they must have the same concentration in both half-cells is also not accurate, as different concentrations can lead to a potential difference, which is the basis for their operation. Lastly, determining cell potential does involve using specific equations, such as the Nernst equation or calculating standard cell potentials from standard reduction potentials, making the statement about not requiring equations incorrect.