Its very simple.
Just look at the graph and see the points where the graph is intersecting the x-axis.Roots are the points where the graph crosses the x-axis.
Hence,
The graph is apparently crossing the x-axis at
x = -1
x = 2
x = 6
So,now it easy to define the domain of the function from the given options.
F. 1.5 to 2.5
C. 5.5 to 6.5
I hope it helped.