Intensity at reflector = I_{1} = P/4p
R^{2}

Power reflected = P_{1} = P= I_{1}A_{1}, where
A_{1} is the area of the reflector.

P_{1} = PA_{1}/4p R^{2}

The intensity at the receiver (same distance) = I_{2} = P_{1}/4p
R^{2} = PA_{1}/16p ^{2}R^{4}

If A_{2} is the area of the receiver, then the power detected
= I_{2}A_{2} = PA_{1}A_{2}/16p
^{2}R^{4} = 1X10^{-10}P

R^{4} = A_{1}A_{2}/16p
^{2}10^{-10} = (p /4)(0.5)^{2}(1)/(16p
^{2}10^{-10}) = 1.243X10^{7}

R = 60 m

Answer is B