Two light sources are said to be coherent if they emit:

Explanation:

Coherent sources of light:

Two narrow sources of light are said to be coherent if they emit waves having:

• The same wavelength (or frequency)
• The same amplitude
• A constant phase relation between them

Conditions required for producing sustained interference:

• The two sources of light must be coherent.
• The sources must be monochromatic (i.e. of a single wavelength).
• For better contrast between maxima and minima of intensity, the amplitudes of the interfering waves should be equal.

So, Two light sources are said to be coherent if they emit waves having constant phase difference.