The graph crossses the x-axis (y=0) at x = -3, x = -1, and x = 2. These are the 3 solutions to f(x)=0 (A is true), and the roots of f(x) (B is true). Since these are all real numbers and there are 3 of them, C is true.
None of these roots is 3, so D is false. The appropriate choice is
... D) Three is a real root of the function f(x).
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A "root of f(x)" and a "solution to f(x)=0" are the same thing. That is what "root" means in this context.
