5. The children (play).........................................noisy when their father came.
Необходимо поставить глагол play в форму Past Continuous.
Past Continuous образуется с помощью вспомогательного глагола was/were + глагол с окончанием -ing.
Получаем предложение: The children were playing noisy when their father came.
Ответ: were playing