Fill in the blanks with the correct modal
Tragedies ______ an episode of depression

may trigger

The correct modal to fill in the blank is may trigger. The modal may is used to express possibility, and it is the most appropriate modal to use in this context because it does not imply certainty. Tragedies can trigger an episode of depression, but they do not always do so.

The other options are incorrect. The modal ought to have triggered suggests that tragedies should trigger an episode of depression, but this is not always the case. The modal should trigger implies certainty, which is not warranted in this context. The modal must trigger also implies certainty, and it is not used to express possibility.