Correct Answer - c
Path difference between light rays coming from two slits
`Delta x =1.8 xx10^(-5)=1.23 xx10^(-5)`
`=10^(-5)(1.85-1.23)=0.57xx10^(-5)=57xx10^(-7)`
`=57000xx10^(-10)=57000 A`
it is not an integral multiple of wavelength thus only dark fringes are possible for nth drk fringe `Delta x = (2n-1)(lambda)/(2)`
`rarr 2n-1 =2xx57/6=57/3=19`
`rarr 2n-1 =19`
or 2n=20 ro n=10
thus 10th dark fringle is formed