Final answer:
The appropriate expression to fill in the blank is 'Es verdad' which means 'It's true'.
Step-by-step explanation:
The appropriate expression to fill in the blank in the sentence 'Que los niños digan la verdad' is A. Es verdad. This translates to 'It's true' in English. This expression is the most appropriate because it directly expresses the idea of the children telling the truth.
Learn more about Appropriate expression to fill in the blank in Spanish sentence