You can choose all those tasks that must be completed before this particular task. Note that it only lists those cards whose titles are formatted like in this example:
(a15) Foundation
In the above example; it denotes the task to be a15 as that is the value given inside the parenthesis. Only cards with such formatted titles can be selected as precedent tasks.
If the above format is not used as the title of a card, then that card would not be used for the critical path analysis.