Diffraction decreases with larger wavelength relative to gap width

Steps:

• Diffraction is most noticeable when wavelength (λ) is comparable to or larger than gap width (d); here, λ = 5 cm, d = 20 cm, so λ < d and diffraction is moderate.
• Wave speed v is constant in the ripple tank, so λ = v / f, where f is frequency.
• Decreasing f increases λ, making λ/d smaller and reducing diffraction.
• Other changes do not alter the λ/d ratio in a way that decreases diffraction.

Why B is correct:

• Decreasing frequency increases wavelength (λ = v/f), reducing the λ/d ratio and thus diffraction, per the diffraction condition for single slits.

Why the others are wrong:

• A: Distance to gap affects wave coherence but not diffraction amount through the gap.
• C: Amplitude affects wave intensity, not the diffraction pattern's angular spread.
• D: Increasing gap width increases d, making λ/d smaller and actually increasing diffraction.

Final answer: B