Diffraction depends on wavelength relative to gap size

Steps:

• Diffraction is most noticeable when wavelength is comparable to or larger than the gap width.
• Wave speed in water remains constant when frequency changes.
• Halving frequency doubles wavelength, since λ = v/f.
• Larger wavelength relative to gap increases diffraction angle and spread.

Why B is correct:

• Doubling wavelength makes waves bend more around barriers, per Huygens' principle, enhancing diffraction.

Why the others are wrong:

• A: Shorter wavelength (higher frequency) reduces diffraction.
• C: Diffraction persists but intensifies with longer wavelength.
• D: Wavelength change alters diffraction extent.

Final answer: B