Final answer:
Normal speech has an intensity level of approximately 60 dB, indicating that the answer to the student's question is option D, 60. This information is supported by understanding various common sound levels in decibels and their everyday comparisons. The correct option is D) 60.
Step-by-step explanation:
The question being asked pertains to the normal sound intensity levels found in conversational speech. Upon examination of the given references and various educational materials, it is determined that the intensity level of normal speech falls around 60 decibels.
This fact is exemplified by the information indicating common sounds at certain decibel levels, where 50 dB equates to the sound level inside a quiet home without any television or radio playing. Therefore, normal speech, being slightly louder than this, would be at an approximate level of 60 dB, option D.
It is important to note the relevance of understanding sound intensity and frequency as they relate to hearing thresholds and the resonance of different environments and sources, such as AC electrical appliances producing sounds at 60 Hz and music typically at 400 Hz.