Based on the relative luminosity curve shown in the figure,...

From the standard relative luminosity curve for photopic vision, the peak relative luminosity is $1.0$ at a wavelength of $555 \text{ nm}$.

As the wavelength increases from $555 \text{ nm}$ to $600 \text{ nm}$, the relative luminosity decreases.

Based on the standard curve (or the specific curve provided in such problems), the relative luminosity at $600 \text{ nm}$ is approximately $0.63$ to $0.66$. Reading directly from the given figure, the value corresponds to $0.66$.