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 [tex] y=\dfrac{1}{3} 9^x [/tex] (violet curve) and the graph of the function [tex] y=-\dfrac{1}{3} 9^x [/tex] (green curve). The second graph is a reflection of the first graph across the x-axis. Then, the correct choice is C.
