What is the radius of moon's orbit around earth assuming the orbit to be circular ($g=9.8 m/s^2$; Radius of earth =6400 km; period of rotation of moon around the earth $=27.3\; days$

Question

$$(a)\;6400\;km \quad (b)\;3.8 \times 10^8\;m \quad (c)\;1000\;km \quad (d)\;10000\;km$$