Which Quick Trip type is valid only for a specific time period after purchase?

The choice of HSD On-board Quick Trip is valid only for a specific time period after purchase because it is designed for immediate use upon boarding the vehicle. This type of fare must be used within a limited timeframe, usually aligning with the time of the trip for which it has been purchased. The design aims to facilitate quick transactions and ensure that riders have a ticket that corresponds to their current trip.

In contrast, other types of Quick Trips may have different usage rules, such as being valid for a longer duration or applicable in more flexible circumstances. For instance, pre-purchased Quick Trips can be bought in advance and used at the rider's convenience. Exit Fare Quick Trips may be used without a strict time limit, calculated based on the route taken. Transfer Quick Trips typically allow riders to switch from one service to another without being time-bound immediately after purchase. Therefore, the HSD On-board Quick Trip's time sensitivity differentiates it and makes it the correct answer for the question.