Which rule in jonas's packet terrifies him, and why?

The rule that terrifies Jonas is eighth on the list, the rule that permits him to lie. No one in the community is allowed to lie, thus, the rule made him wonder if others had been told they could lie as well.

Thank you, I really want to just enjoy the book and not have to worried about the questions I have to answer.

If you need to cite evidence for this, then the paragraph you need is page 71 of chapter 9, which states "Now Jonas had a thought that he had never had before. This new thought was frightening. What if others — adults— had, upon becoming Twelves, received in their instructions the same terrifying sentence?"