Final answer:
The idea that shapes "A Wagner Matinée" is the transformative power of music, which deeply affects the characters' emotions and memories.
Step-by-step explanation:
The idea that most clearly shapes "A Wagner Matinée" is c) The power of music. This piece of literature illustrates how music can evoke deep emotional responses and can transport individuals back to previous times in their lives, reminding them of past joys, sorrows, and the love for the arts that they once had. Set against the backdrop of city life, the story highlights the transformative and sometimes overwhelming effect that music, especially the complex and emotional scores of composers like Wagner, can have on individuals.