If light waves are coherent, a. they shift over time. b. their intensity is less than that of incoherent light. c. they remain in phase. d. they have less than three different wavelengths.

The correct option is: (c) They  remain in Phase

Two waves are coherent if the following three conditions are met:
1) They have the same frequency.
2) They have the same waveform. 
3) They remain in phase.

Hence the correct option is C.

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