In this case constructive interference will occur and it will sum up or add both waves together. So that the resulting wave will travel at speed of 4.2 hertz. So, the correct option is C.
The correct answer to the question is the resulting wave would travel at 4.2 hertz. Wave A <span>traveling at 3 hertz, crash to Wave B which is traveling at 1.2 hertz causes a constructive interference resulting to summing up their speed.</span>