Final answer:
The potential difference across the 3 cells that are joined in series is additive.
Step-by-step explanation:
The potential difference across the 3 cells that are joined in series is b) The potential difference is additive. When cells are connected in series, the total potential difference is equal to the sum of the individual potential differences of each cell. This means that the potential difference across the 3 cells will be the sum of the potential differences of the individual cells.