C. 50Hz. The wavelength of a wave on a string is 1.2m with a speed of 60m/s its frecueny is 50Hz.
The wavelength of a wave describes how long the wave is, and can be represented by the formula λ = v/f, where λ is the wavelenght in meters, v is the speed of the wave in m/s, and f is the frecuency of the wave in Hertz.
The wavelength of a wave on a string is 1.2m and its speed is 60m/s, to calculate its frecuency we have to clear f from the equation λ = v/f. So:
f = v/λ
f = (60m/s)/(1.2m)
f = 50 1/s = 50Hz