Answer: Option C
:
C
Given:
Distance between source and observer, $s=1000m$
Velocity of sound, $v=340m{s}^{-1}$
Frequency of sound, $f=500Hz$
Time taken for the sound wave to reach the observer from the source is:
$t=\frac{s}{v}$
Number of oscillations in $1s$ is the frequency, $f$ 
Number of oscillations in time $t$ will be: 
$n=tf$
$n=\frac{s}{v}f$
$n=\frac{1000\times 500}{340}$ 
$n\approx 1470$