Nico vibrates an object at different frequencies. at a certain frequency, the sound gets louder and then returns to the same volume when he moves to the next frequency. what is occurring when the sound gets louder? check all that apply. the natural frequency of the object is changing. the amplitude of the wave is doubling. a standing wave is being generated. resonance is creating the louder sound. the material is vibrating at its natural frequency.

First and foremost, the problem specifies that it operates at a CERTAIN frequency. So, the natural frequency is not changing. Consequently, if frequency does not change, amplitude doesn't also. Next, a standing wave is produced when two waves move at opposite directions. Since it wasn't specified, this is not entirely true. Thus, the statements that apply are the last two statements:

Resonance is creating the louder sound.
The material is vibrating at its natural frequency.