Answer:
D3
Step-by-step explanation:
To determine the note, you will have to find the frequency of that sound wave
frequency is no. of cycles or oscillation that occurs per second
so according to the question for 3 oscillations, it took 20ms which is 20 × 10^-3 seconds then for 1 second that'd be
3×10^3 ÷ 20 = 150 Hz
then the note corresponding to frequency is D3