Solution:
Concept- Charges have the property that they can flow from a low voltage direction to a high voltage direction. And we should also know that when a short circuit occurs, a very large amount of current flows.When two batteries have different voltages, they cannot be connected in parallel. The low voltage battery is being charged by the high voltage battery if we link them in parallel. Therefore, there is a possibility of harming that circuit. However, if we connect them in series, the two voltages will be added and we'll get that extra voltage at the output side. Therefore, two batteries with different voltages can only be linked in series.
Hence, the correct option is (3).
