A drone fitted with siren in flying directly away from the drone operator and towards a distant building at a speed of 15 m/s . The siren produces the sound of frequency 780Hz. What is the frequency that the operator hears in the echo refected from the building,[ speed of sound is 340 m/s]

**Answer:**

Option C**Explanation:**

A drone fitted with a siren as shown in the figure,

Given, speed of drones , $v_{s}=15 m/s$ , actual frequency , $n_{0}=780 Hz$ and speed of sound , v=340 m/s

As source is moving towards the building, so the apparent frequency reflected by the building is

$n= \frac{n_{0}v}{v-v_{s}}= \frac{780 \times 340}{340-15}=816 Hz$

As, we know there is no change in the apparent frequency by the reflection from the building Hence,

the echo heard by the observer is 816 Hz.

So, the correct option is (c)