Explanation

(c) The apparent change in the frequency of the source due to a relative motion between the source and observer is known as Doppler’s effect. The perceived frequency is given by
$v^{'} = v\left( \frac{v - v_{o}}{v - v_{s}} \right)$
Where v is original frequency, v the speed of sound, $v_{o}$ speed of observer, $v_{s}$the speed of source. In the given case there is no relative motion between source and observer, since both are at rest, hence frequency of sound heard by the observer will remain unchanged.