Question

What prompts huck to tell the truth in this part of the story? he feels it is the right thing to do. he feels it is the safest thing to do. he wants the reward he will gain from it. he wants the con artists to get into trouble?

Answer 1

The safest thing to do

Answer 2

Assuming that you're referring to the kag of powder scene, the thing that prompts huck, to tell the truth in this part of the story is: He feels it is the right thing to do.
He feels that The truth is always better and safer( he had various bad experiences of being punished because he was caught up in a lie)