Explanation

(d) Using red light $(\lambda = 6600\mathring{\mathrm{A}})60$ fringes are seen.
Hence, range of field of view is
$\Rightarrow 60 \times w = 60 \times \frac{D\lambda}{d}\ $
Using light of wavelength $\lambda^{'},\ n$ fringes are seen, then
$\Rightarrow 60 \times \frac{D\lambda}{d} = n \times \frac{D\lambda^{'}}{d}$
$60 \times \lambda = n \times \lambda'$
$\Rightarrow n = 60 \times \frac{\lambda}{\lambda^{'}} = 60 \times \frac{6600}{4400} = 90$