A stationary (standing) wave is formed when:

  1. one wave is reflected and absorbed
  2. a wave stops moving
  3. two waves of different frequencies meet
  4. two waves of the same frequency and amplitude travel in opposite directions and superpose
Answer

D) two waves of the same frequency and amplitude travel in opposite directions and superpose

Superposition of opposite travelling waves. SLO P-11-D-11 · Understanding

Physics MCQ · More Superposition, Stationary Waves and Beats MCQs · Practise