The correct answer is: A. increase the voltage rating.
Cells are connected in series to increase the voltage rating. When cells are connected in series, the voltage of the combined cells is equal to the sum of the voltages of the individual cells. For example, if you connect two 1.5-volt cells in series, the combined voltage will be 3 volts.
The other options are incorrect. Option B is incorrect because connecting cells in series does not increase the current rating. The current rating of a cell is determined by the internal resistance of the cell. Option C is incorrect because connecting cells in series does not increase the life of the cells. The life of a cell is determined by the chemical reactions that occur inside the cell.
I hope this helps!