Answer:
A) Energy
Explanation:
Energy and Amplitude are directly proportional because an increase in energy always results in an increase in amplitude, and vice versa, so they are directly proportional. As for the other options, they are all independant from the amplitude (i.e. Changing them would not result in a change in amplitude e.g. A wave can have a big amplitude and high frequency or a big amplitude and low frequency, as neither affects the other). Therefore it is option A) Energy.
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