A light source produces monochromatic light of wavelength 555 nm ....

The relative luminosity of light of wavelength $555 \text{ nm}$ is $1$.

The luminous flux corresponding to $1 \text{ W}$ of radiant flux at $555 \text{ nm}$ is $685 \text{ lumen}$.

Total luminous flux emitted by the source is given by:

$\Phi_v = 35 \text{ W} \times 685 \text{ lumen/W} = 23975 \text{ lumen}$

The overall luminous efficiency is defined as the ratio of the total luminous flux to the total electric power consumed by the source.

Overall luminous efficiency $= \dfrac{23975 \text{ lumen}}{100 \text{ W}} = 239.75 \text{ lumen/W}$

Rounding off to the nearest integer, we get $240 \text{ lumen/W}$.