Intensity at reflector = I1 = P/4p R2
Power reflected = P1 = P= I1A1, where A1 is the area of the reflector.
P1 = PA1/4p R2
The intensity at the receiver (same distance) = I2 = P1/4p R2 = PA1/16p 2R4
If A2 is the area of the receiver, then the power detected = I2A2 = PA1A2/16p 2R4 = 1X10-10P
R4 = A1A2/16p 210-10 = (p /4)(0.5)2(1)/(16p 210-10) = 1.243X107
R = 60 m
Answer is B