Which rule in his packet terrifies Jonas?

In chapter nine, Jonas reads through the rules he has received as the new Receiver of Memory. Although there are only 8 rules, he finds a couple of them disturbing. Rule 6 scares him because he has never truly experienced pain and is afraid of the possibility that he will not be able to take any medication to relieve whatever pain he feels.
The rule that is most scary to him is the final rule, number 8. Rule 8 states, "You may lie." In Jonas's community, being honest and upfront is considered a sacred rule. People do not lie to each other, nor do they make each other upset or uncomfortable without immediate apology. Jonas's community trains each citizen from a very young age to use Precision of Language and lying goes against that practice, even if it is unintentional. "Once, when he had been a four, he had said, just prior to midday meal at school, "I'm starving." Immediately he had been taken aside for a brief private lesson in language precision. . . To say "starving" was to speak a lie. An unintentional lie, of course."
At the end of the chapter, we see Jonas grapple with his changing perception. He believes that no one in the community lies, but what if they were all allowed to lie? Then even if he were to ask them whether or not they were allowed to lie, he would never truly know if they were telling the truth.

In chapter 9, Jonas reviews the rules for his assignment, and he is unnerved about rule number six, which forbids him from taking any medication related to his training. Jonas remembers that the Chief Elder had mentioned that he would experience an extreme amount of pain. During the Ceremony of Twelve, the Chief Elder also told Jonas that the magnitude of pain he would experience would be greater than anyone in the community ever endures, which is why his assignment requires courage. Medication is always readily available to the community members, and Jonas shudders at the thought of experiencing immense pain without using medication for relief. When Jonas remembers that the Chief Elder used the word "indescribable" to explain the pain, he swallows hard and tries not to think about it.