Diffraction maxima separation formula Steps:

• The angular position of maxima is θ_m ≈ mλ/d for small angles, where d is slit separation.
• Linear separation on screen Δy ≈ L λ / d, with L as screen distance.
• To decrease Δy, decrease λ or L, or increase d.
• Option D increases d (fewer slits per unit length means larger d), decreasing Δy.

Why D is correct:

• Larger slit separation d reduces the diffraction angle θ = λ/d per the grating equation, shrinking maxima separation.

Why the others are wrong:

• A: Larger L increases linear separation Δy proportionally.
• B: Smaller d increases θ = λ/d, widening separation.
• C: Red light has longer λ than green, increasing θ and Δy.

Final answer: D