Question

In a mobile phone network, how many times as strong would a signal be expected to be at 500 m from a base station, compared with at 250 m? Choose the nearest value from the list below.

Answer 1

It depends on the unit of measure you are using for "as strong"...power intensity, Electric field intensity, Magnetic Field intensity. 

And, there are logrithmic measures for all of these two. 
But if you consider just energy spreading, the Power density= Power/area 
and area is proportional to radius^2 

so the power intensity change is 1/(1/3)^2=9