To achieve an interference pattern, we must have two coherent wave sources. For coherence, the waves must have the same frequency and either have no phase difference or a constant phase difference.
For two waves traveling in the same medium, having the same frequency implies having the same wavelength by v = fλ where v = velocity, f = frequency, and λ = wavelength.
An interference pattern requires all of the above conditions except for having the same amplitude.
Choice C