Final answer:
The appropriate expression to fill in the blank is 'sin darse cuenta' which means 'without realizing' in English.
Step-by-step explanation:
The appropriate expression to fill in the blank is sin darse cuenta which means 'without realizing' in English. In this sentence, the professor gave the students the exam answers unintentionally. This phrase indicates that the action was done without the professor's awareness.
Learn more about Spanish expressions