ANSWER
d. Subaudible
Step-by-step explanation
Sound intensity is usually measured in decibels.
The audible sound ranges from 0 dB to 120 dB while any sound that is beyond 120 dB is regarded as harmful and can cause hearing impairment.
Since the decibel is not an absolute measure of sound energy but a comparison with a value known as the reference value, it is possible to have negative decibels.
Therefore, a negative decibel means that the sound is quite lower or softer than this reference value or threshold.
Hence, the answer is subaudible noise.