Concept:The superposition of two sound waves of nearly equal frequencies produces a periodic variation in loudness called beats.
Explanation:When two notes of slightly different frequencies are sounded together, their waves alternately reinforce and cancel each other.
This causes a periodic rise and fall in the intensity of sound, which is known as a beat.
The beat frequency equals the difference between the two frequencies,
fbeat=∣f1−f2∣.
Doppler effect is the change in observed frequency due to relative motion between source and observer.
Interference is the general combination of waves, while resonance is the increase in amplitude when frequency matches the natural frequency.
Therefore, the specific phenomenon described is beats.
Answer:B. beat