When cells are connected in series, the total electromotive force (emf) of the battery is the sum of the individual cell voltages. This assumes all cells are oriented correctly, with positive to negative in a chain. If a cell is connected in the opposite direction, its voltage subtracts from the total rather than adds to it. In this problem, we have a total of 12 cells, each with an emf of 5V. Normally, if all were connected correctly in series, the total emf would be: Total emf = Number of cells × emf of each cell Total emf =12×5V=60V However, 3 cells are connected incorrectly, which means their voltage contribution would be negative instead of positive. So, for these 3 cells, instead of adding 3×5V, we subtract 3×5V : Adjusted total emf = Total emf without errors −2× (emf of 3 wrong cells ) Adjusted total emf =60V−2×15V Adjusted total emf =60V−30V=30V Therefore, the resultant emf of the battery, taking into account the 3 incorrectly connected cells, is 30V. This corresponds to Option B.