In this opinion question, you'll want to decide for yourself what you feel about Marty's lie, then find textual quotationsto support your opinion.
For instance, if you feel Marty did the right thing by keeping Shiloh and lying to his father, you will highlight the parts of the book that tell of Marty's kindness and help in bringing Shiloh back to good health. You will emphasize that Marty was acting as he did to protect part of God's creation.
On the other hand, if you decide what Marty did was wrong, you will point out the consequences Marty faced with Judd Travers and with his father as a result of his lies. You will discuss how Marty felt about not living in a completely truthful relationship with his family.