These are the choices to your question:
A. velocity=frequency/wavelength
B. wavelength=velocity*frequency
C. velocity=frequency*wavelength
D. frequency = velocity*wavelength
A way to check for the correct formula is by its units. So, going with this, wavelength is meters per second, which is precisely equal to the units of velocity, which in this case is in meters per second. So, among all the choices, all are incorrect statements of the wave-speed equation except for velocity=frequency*wavelength or letter C.