Answer
1/s or $s^{-1}$ (s is for second)
Work Step by Step
The question asks for the SI unit for frequency of light. Frequency of light measures the number of cycles (complete wavelengths) passing a point each second, the SI unit for it is 1/s or $s^{-1}$.
You can also deduce the SI unit for the frequency of light from this important formula, if you already know the SI units for wavelength of light and speed of light:
$\lambda\times\nu = c$
$\lambda$ (lambda): wavelength of light (m)
c: speed of light (m/s)
