Q.Define Coherent source.

A clean, visible double-slit (or any) interference pattern requires: (1) coherent sources -- the one essential requirement, without which no stable pattern survives; (2) monochromatic light -- since fringe positions and width both depend on $\lambda$, mixed wavelengths (white light) give overlapping, smeared, coloured fringes rather than sharp ones; (3) equal amplitudes from both sources -- needed for dark fringes to reach true zero intensity, i.e. maximum contrast; (4) $D\gg d$ -- needed for a fringe width large enough to actually observe; …