Ultrasound has a frequency of vibration-
$\left( a \right)$ Between $20Hz$and $20000Hz$
$\left( b \right)$ Below $20Hz$
$\left( c \right)$ Above $20000Hz$
$\left( d \right)$ Between $500Hz$and $10000Hz$

Hint So here to answer this specific question we should have to study the different types of vibration and also we have to know the different ranges of the frequencies which will depend on the frequencies.

Complete Step by Step Solution Human ears can't detect sound frequencies under twenty vibrations for each second for example $20Hz$ for every second. Thus, any vibration underneath that recurrence will be indistinct to people. The human ear can't detect frequencies over $20000$ vibrations for every second $20Hz$ in the high-frequency range, and the adequacy of the wave will be subject to the sound's clamor. Consequently, the frequencies under $20Hz$ and more than $20kHz$ fall under the indistinct frequencies band. The unintelligible higher frequency range is otherwise called ultrasonic sound.
Subsequently, stable waves having frequency more than $20000Hz$ are called ultrasonic sounds.

Therefore, the option $\left( c \right)$will be correct.

Additional information We can make ultrasound utilizing electrical gear that vibrates with an amazingly high recurrence. Gems of certain materials, (for example, quartz) vibrate quickly when we give power through them—an impact called piezoelectricity. Gadgets that produce ultrasound waves utilizing piezoelectricity are known as piezoelectric transducers. Piezoelectric precious stones additionally work oppositely: if ultrasound waves going through the air end up crashing into a piezoelectric gem, they crush its surface somewhat, making a short eruption of power move through it. Thus, if we were up a piezoelectric precious stone to an electric meter, we get a moment ultrasound identifier.

Note They are used in multiple applications, although the technology is still in infant stages, like welding, soldering, SONAR (exists as a major technology), etc. the misnomer ultrasound was coined because it is the highest threshold of human hearing censoring.