A monochromatic source of 1 W produces a luminous flux of 450 lumen...

The maximum luminous flux for a $1 \text{ W}$ source is $683 \text{ lumen}$ (which occurs at a wavelength of $555 \text{ nm}$).

The relative luminosity is defined as the ratio of the luminous flux at the given wavelength to the maximum luminous flux for the same power.

Relative luminosity $= \dfrac{450}{683} \times 100\%$

Relative luminosity $\approx 65.88\% \approx 66\%$