Final answer:
The sound becomes quieter because the amplitude decreases, a result of damping. Damping causes the energy of the sound to spread over a larger area and be absorbed, thereby reducing the sound's loudness. Therefore, the correct option is D.
Step-by-step explanation:
The correct answer to the student's question, 'The sound is becoming _____ due to _____,' is:
D. The sound is becoming quieter because the amplitude is decreasing.
The amplitude of a sound wave is directly related to its loudness; a larger amplitude means a louder sound, while a smaller amplitude means a softer sound. The reason why sound becomes quieter as it travels is primarily due to damping. Damping occurs as the energy of the sound wave is spread over a wider area and absorbed by materials in the environment, converting it to other forms of energy such as thermal energy. This physical principle is explained by the second law of thermodynamics.
The increase or decrease in amplitude is not affected by the wavelength or frequency of the sound wave, which are related to the pitch of the sound.