10. One sentence has mistakes. Find it. • I had explained everything to my friend before I asked for his advice. • The children wondered who was playing the piano next door. • Pauline had taken out her mobile, dialed the number and heard the busy signal

Разбираемся:

• Первое предложение верно: «I had explained everything to my friend before I asked for his advice.» (Past Perfect и Past Simple использованы корректно).
• Второе предложение верно: «The children wondered who was playing the piano next door.» (грамматически всё верно).
• Третье предложение содержит ошибку в согласовании времен: «Pauline had taken out her mobile, dialed the number and heard the busy signal». Необходимо использовать Past Simple во всех глаголах:

Исправленный вариант:

• Pauline took out her mobile, dialed the number, and heard the busy signal.

Ответ: Ошибка в третьем предложении.