The correct option is C 3 cells connected in series
In the electric source, the more the number of cells connected in series, the brighter the bulb glows. So 3 cells connected in series would result in more brightness of the bulb than the 2-cell case. On the other hand, adding any number of cells in parallel does not affect the brightness of the bulb. So 4 cells in parallel and 5 cells in parallel would result the same brightness as one cell connected to the bulb. Cells in parallel would last longer as compared to a single cell.
Hence, 3 cells in series would result in the brightest bulb.