Thank you so much!!!!! for the three answers, gosh I really need to pay more attention when I do the papers I did not see the word "visible" until now.You apply v=f(lamda) to find the frequency. You know the wavelength of the visible spectrum ranges from 400nm to 700nm, and that the velocity of electromagnetic waves is 3x10^8.
So 3x10^8=400x10^-9(f)
The f turns out to be 7.5x10^14 Hz. Hence, the order of frequency is 10^14 Hz(C).