Final answer:
The sound that measures 65 dB is equivalent to 65 phons, assuming it is at 1000 Hz frequency where the phon and dB scales are the same. Therefore, the 50 Hz sound that measures 65 dB would be 65 phons, which is option B).
Step-by-step explanation:
A sound that measures 65 dB on a sound meter is equivalent to 65 phons at 1000 Hz frequency, since the phon scale is based on the perceived loudness relative to a 1 kHz tone. However, if the frequency is not 1000 Hz, one would need to refer to equal-loudness contours to convert dB to phons since the sensation of loudness changes with frequency.
Because the question does not specify the frequency or provide an equal-loudness contour chart, we must assume that the sound in question has a frequency of 1000 Hz. Therefore, the 50 Hz sound that measures 65 dB would be 65 phons, which is option B).