Answer:
Because Taylor is wondering if he can give Turtle the best possible life, but that is impossible. In that case Mattie says the right question is not whether Matie can give Turtle a good life, but whether she is willing to try.
Step-by-step explanation:
You have not submitted the text to which this question is related. However, by looking at the characters' names and the relationship between them, presented in the question above, we can see that this question is about "The Bean Trees" that tells the story of Taylor, who decides to abandon her local city and embark on a journey of self-discovery. On this journey, she ends up adopting Turtle, a Cherokee boy.
Taylor wants to protect Turtle and give him the best possible life and the most efficient education, but she wonders if she can do it. In this context, Mattie tells her that this question is incorrect, because no one is able to give the best possible life to a person. For Mattie, the correct question Taylor should ask is if she is willing to try to give Turtle the best possible life.