Question

Which graph represents a reflection of f(x) = 1/3 (9)x across the x-axis?

Answer 1

Hey there...
Thats easy
Reflection across the x axis means it is a Vertical reflection. 
the correct choice is c

Answer 2

The reflection across the x-axis means that point with coordinates (x,y) after reflection will have coordinates (x,-y).

In attached diagram you can see the graph of the function $y=\dfrac{1}{3} 9^x$ (violet curve) and the graph of the function $y=-\dfrac{1}{3} 9^x$ (green curve). The second graph is a reflection of the first graph across the x-axis. Then, the correct choice is C.