Fringe spacing in double-slit interference Steps:

• Recall the formula for fringe spacing: β = λD/d, where λ is wavelength, D is slit-to-screen distance, and d is slit separation.
• Identify apparatus distances: p as slit-to-screen distance (D), q as slit separation (d), r as source-to-slit distance.
• To increase β, decrease d while keeping λ and D constant.
• Decreasing q reduces d, directly increasing β.

Why B is correct:

• Decreasing q decreases slit separation d, and since β = λD/d, smaller d increases fringe spacing per the interference formula.

Why the others are wrong:

• A: Decreasing p decreases D, which decreases β via β = λD/d.
• C: Decreasing r has negligible effect on β, as source distance doesn't directly alter the formula.
• D: Decreasing wavelength λ decreases β via β = λD/d.

Final answer: B