The main idea is that there is a permissible on-ground extension to the Flight Duty Period, but it’s tightly controlled to protect fatigue. You can extend the FDP by up to two hours on the ground, but you must stay within a rolling limit of 168 hours of FDP in any 672 consecutive hours (a 28-day window). This means the extra ground time is allowed only if your total FDP hours over the past 28 days don’t exceed that 168-hour cap. It gives crews flexibility to handle ground delays while preventing excessive duty. The other options miss or contradict this balance: there is an allowed ground extension, it isn’t conditional on the flight ending early, and the restatement in the other choice rounds exactly the same ground-extension concept but without altering the key limitation.