Final answer:
(A) True
Cation-sensitive electrodes, such as a calcium ion-selective electrode, can accurately measure the concentration of calcium ions in the presence of sodium, making the statement true.
Step-by-step explanation:
A cation-sensitive electrode, such as a calcium ion-selective electrode, is specifically designed to measure the concentration of a particular species in solution, such as calcium ions.
These electrodes, including those for calcium, are based on the principle where the potential of the electrode depends solely on the concentration of the desired ionic species in solution, despite the presence of other ions such as sodium.
Therefore, the correct answer to whether a cation-sensitive electrode is used to determine the activity of calcium in the presence of sodium is A) True.