# Two light sources emit light of same wavelength $\lambda$. They will form interfering patterns if

(a)They have same intensities

(b)They have same frequencies

(c)They have constant phase difference

(d) phase is changing randomly

For interference pattern to term the two light waves must be coherent ie they must have a constant phase difference between them. The intensities need not be same. If phase difference keeps on changing no steady pattern will be formed.
Hence c is the correct answer.
edited Mar 5