Diffraction requires wavelength comparable to gap size Steps:

• Define diffraction as the spreading of waves through a narrow gap or around obstacles.
• Recall Huygens' principle: waves propagate via secondary wavelets that interfere, prominent when gap ≈ wavelength.
• Identify key condition: for noticeable diffraction, wavelength must be long relative to gap width.
• Eliminate unrelated factors like medium or amplitude.

Why D is correct:

• According to the diffraction condition, significant bending occurs when wavelength λ ≥ gap size d, so long λ enables wavelets to spread and overlap effectively.

Why the others are wrong:

• A: Not required; mechanical waves (e.g., sound, water) diffract in media without vacuum travel.
• B: Progressive (traveling) nature is needed for propagation, but "in a vacuum" limits to EM waves; mechanical waves diffract too.
• C: Amplitude influences intensity but not diffraction occurrence or pattern shape.

Final answer: D