The laser light beamed towards moon from earth takes $2.56s$ to return after reflection at moon's surface. How much is the radius of lunar orbit around earth?

Question

$$(a)\;7.84 \times 10^8 m\quad (b)\;6.84 \times 10^8 m \quad (c)\;2.84 \times 10^8 m\quad (d)\;3.84 \times 10^8 m$$