An electrochemical cell is an electrolytic cell that drives a non-spontaneous redox reaction through the application of electrical energy. This cell is used to decompose chemical compounds, in a process called electrolysis. An electrode at which reduction take place is called the cathode. In reduction, electrons travel toward the site of reduction such that the negative charge is on the cathode.
The correct answer would be D. Ca^2+ has 18 electrons. This is because 2+ means that 2 electrons have been removed, and Ca starts out with 20 electrons. As 20 - 2 = 18 electrons, D is the correct answer.
