Creating a database structure for a quiz
The correct answer is the property of the question.

Thus, the question table should be improved by a column named correct_answer_id, which will be a foreign key to answer(id_answer).

We can see some similar in the database of the Stack Exchange - here the accepted answers are in the AcceptedAnswerId column.

This structure has a possibility of inconsistence - nothing guarantees that the correct_answer_id refers to an answer which belongs to the current question. Ideally, you should use contraints to ensure that.