Answer :
We are to get the equivalent expression to the function when x is not equal to - 3.
[tex]\frac{x^2-9}{x+3}[/tex]First, we try to factorize the numerator, we can see that it is a difference of two squares, so we can factorize it;
[tex]\begin{gathered} \frac{x^2-9}{x+3}=\frac{(x-3)(x+3)}{x+3} \\ \text{the x+3's cancel out to give} \\ (x-3) \end{gathered}[/tex]Therefore, the correct answer is option D.