Interference is a condition that occurs when
a) one side of a wave moves at a different speed than the other
b) a wave encounters an obstruction
c) two or more waves overlap and combine
d) the source of the wave’s vibration loses energy

For the answer to the question above, To set up a stable and clear interference pattern, two conditions must be met: The sources of the waves must be coherent, which means they emit identical waves with a constant phase difference. So I think the answer is c.two or more waves overlap and combine.
Because waves needs to be monochromatic or in single wavelength

Sound energy is produced when an object vibrates. examples are 1- a balloon popping2-a whistle3- a waterfall