When a piano tuner hears a beat frequency, it indicates that the frequency of the tuning fork and the frequency of the piano wire are slightly different. To reduce the beat frequency, the tuner needs to adjust the tension in the wire so that the frequency of the wire matches more closely with that of the tuning fork.
Increasing the tension in the wire will raise its frequency, while decreasing the tension will lower its frequency. To decrease the beat frequency, the tuner will want to decrease the difference between the two frequencies.
Therefore, the correct response is:
decrease the amount of interference and decrease the beat frequency.