In Y.D.S.E select the correct choice

    a)    The two slits should be close to each other
    b)    If slits are close, this ensures small path difference
    c)    If slits are close, it ensures large path difference
    d)    Overlapping of fringes is avoided if slits are closer.

In Y.D.S.E rays emerging from the slits should be nearly parallel. For that the condition D >> d should be satisfied. So slits should be closer. Since path difference is d sinθ, closer slits ensure small fringe width

for smaller d, β increases and fringe pattern becomes more distinct and overlapping can be avoided.

So Option -(3) is right.