Cells are connected in parallel to increase the current capacity. When cells are connected in parallel, the current capacity of the system increases by the number of cells connected. For example, if two cells are connected in parallel, the current capacity of the system is doubled. This is because the current flowing through each cell is the same, but the total current is the sum of the currents flowing through each cell.
The other options are incorrect. Option A is incorrect because the efficiency of a cell is not affected by whether it is connected in parallel or in series. Option B is incorrect because the voltage output of a cell is not affected by whether it is connected in parallel or in series. Option C is incorrect because the internal resistance of a cell is not affected by whether it is connected in parallel or in series.