Two sources are called coherent if(A) Both have same amplitude of vibration(B) Both produce waves of the same wavelength(C) Both produce waves of the same wavelength having constant phase difference(D) Both produce waves having the same velocity

Two sources are called coherent if:

(C) Both produce waves of the same wavelength having a constant phase difference.

Coherent sources produce waves that maintain a constant phase relationship with each other, which means they have the same frequency (same wavelength) and their waveforms are synchronized, or they have a constant phase difference between them. This is important in various optical and wave phenomena, such as interference and diffraction.