Which rule prohibits passing on upgrades or downgrades on two-lane roads?

Answer: C — Two-Lane Highway Rule

Passing on uphill or downhill sections of a two-lane road is not allowed because your sight distance is limited and you can’t safely complete a pass before meeting oncoming traffic. The rule that covers this situation is the Two-Lane Highway Rule, which specifically prohibits passing on upgrades or downgrades on two-lane roads to prevent head-on collisions. This is about safe passing in the unique visibility and speed conditions created by grades, not about braking systems, descent speeds, or signaling rules.