Final answer:
Correct option: (c) Beat interference
The phenomenon caused by the superposition of two sound waves of slightly different frequencies with zero initial phase difference and the same amplitude, resulting in the fluctuation of loudness, is known as beat interference.
Step-by-step explanation:
When two sound waves of slightly different frequencies f1 and f2 with zero initial phase difference, and the same amplitude A, travel in the same direction, the resulting phenomenon is known as beat interference. This happens because the two sound waves periodically reinforce and diminish each other, leading to an oscillation in the loudness of the sound, which is perceived as a beat. The beat frequency is equal to the absolute difference between the two frequencies, i.e., |f1 - f2|.
The options given: (a) Constructive interference occurs when two waves are in phase and their amplitudes add together. (b) Destructive interference occurs when two waves are out of phase and their amplitudes subtract from each other. (d) Resonance interference is not a standard term used in the context of interference of sound waves. Therefore, the correct answer is (c) Beat interference.