Stationary waves form via superposition of oppositely traveling coherent waves

Steps:

• Identify conditions for stationary waves: two waves must superpose with fixed interference pattern.
• Require same frequency (period) and wavelength for nodes/antinodes to remain stationary.
• Waves must be coherent and of same type for effective interference.
• Direction must be opposite; same direction yields progressive wave, not stationary.

Why D is correct:

• Stationary waves demand opposite directions for phase differences creating fixed nodes, per superposition principle.

Why the others are wrong:

• A: Different types (e.g., transverse vs. longitudinal) prevent proper interference.
• B: Unequal wavelengths cause moving beats, not fixed pattern.
• C: Different periods imply unequal frequencies, disrupting stationary nodes.

Final answer: D