d
Actual frequency willbe heard when velocity of motor cyclist is perpendicular to line joining the source and motor cyclist is perpendicular to line joining the source and motor cyclist. If will happen $2$ times per round.
No. of rounds in a hour $=\frac{3600}{4 \pi}$
(where $4 \pi$ is time for one round) $\approx 287$
Answer is double of $287$ ie $574$