A rocket is moving at a speed of 220m/s towards a stationary target, emits a sound of frequency 1000Hz, some of the sound reaching the target gets reflected back to the rocket as echo. The frequency of the echo as detected by the rocket is (Take velocity of sound  = 330m/s)

Here, the Doppler effect is used to explain how frequency changes occur as an observer and a source move closer to or farther away from one another. As one approaches a blowing horn, for instance, the perceived pitch is greater until one reaches the trumpet and then drops as one passes the horn.

Frequency of the sound is $f=1000Hz$
Speed of the rocket is $v_R=220m{s}^{-1}$
Speed of the rocket is equal to the speed of the sound.
Speed of the sound is $v_s=220m{s}^{-1}$
Velocity of the sound is $v=330m{s}^{-1}$
Let the frequency of the echo as detected by the rocket is $f^{\prime }$
$f^{\prime }=f\left({\displaystyle \frac{v+v_R}{v-v_s}}\right)$
Place the given values in the above equation –
$f\text{'}=1000\left(\frac{330+220}{330-220}\right)$

$f\text{'}=1000\left(\frac{550}{110}\right)$

$f\text{'}=1000\left(5\right)$

$f\text{'}=5000 Hz$

Hence, option C is correct.