Answer: A
Explanation: The relation between velocity and frequency of the waves is:
f = v/λ
where f is frequency, v is the velocity and λ is the wavelenght.
So you can see that, if the 4 waves have the same velocity, then the one with the smallest wavelenght (because it is in the denominator) will have the highest frequency.
The one with the smallest frequency is the first one (you can see it) so the correct option will be A