Answer: Hello mate! 
we have the function f(x) = 4(1/2)x, and we want to reflex it over the x-axis. 
you can see in the graph that the reflex over this axis changes the sign of f(x) in all the points (where f(0) = 4, g(0) = -4, f(1) = 2, g(1) = -2, and so on), then the reflex, g(x) is equal to -f(x) 
now we have: 
g(x) = -f(x) = - 4(1/2)x 
then the right answer is the third option: 
g(x) = -4(1/2)x