(c) : For interference, by Young’s double slits, the path difference xd/D= nλ for bright fringes and xd/D = (2n +1)λ/2 for getting dark fringes.
The central fringes when x = 0, coincide for all wavelengths.
The third fringe of λ1 = 590nm coincides with the fourth bright fringe of unknown wavelength λ.
∴ xd/D = 3 × 590 nm = 4 × λnm
∴ λ = (3 × 590)/4 = 442.5nm.