we studied FIFO, Priority, Round Robin (RR), and Weighted Fair Queueing (WFQ) packet scheduling disciplines. Which of these queueing disciplines ensure that all packets depart in the order in which they arrived?

FIFO stands for First In First Out. In this type of handling approach the oldest (first) entry, or 'head' of the queue, is processed first.

Round Robin assigns time slices to each process in equal portions and in circular order and there is no priority in the processes,

FIFO is the one that ensures that all packets depart in the order in which they arrived.