A Deque (or LinkedList specifically) can be used for either FIFO or FILO. It's not only FIFO or FILO, but it supports both equally well.
A PriorityQueue has nothing to do with FIFO or FILO. Rather, the order (the priority) is determined by a Comparator (or the Comparable implementation). So it's inappropriate to use a LinkedList or other Deque if you need a priority queue, and it's inappropriate to use a PriorityQueue if you need either FIFO or FILO.
What I understood is that for the Queue (and possibly the Deque?) abstract data structures we are supposed to know the ArrayDeque and LinkedList implementation classes.
As far as I know, the PriorityQueue class is not on the exam (although the more we know, the better of course).
I suggest you have a little look at the Java™ Tutorials, particularly the first part of that section. Also find an introduction to data structures (book or tutorial).
The best type of collection depends mostly on how you are going to use the data in it.
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