Read the given sentence and choose the correct modal auxiliaries to fill in the blanks.
Mary tried to finish her work quickly so that she ______ leave early.

The correct answer is: Can

Option b is eliminated since it expresses something that has already happened in the past and the sentence talks about Mary's actions affecting the future. Option c expresses certainty, which is not quite appropriate because Mary has not finished her work as of yet- so it is not confirmed whether she will be leaving or not.
Thus option a, which expresses possibility, is the correct answer.