The answer is A: Grete is playing the violin and Gregor wants to tell her how beautiful it sounds.
In The Metamorphosis, by Franz Kafka, Gregor Samsa finds himself one day transformed into a bug. As the story progresses, Gregor struggles to become accustomed to his new identity and the kind of life he is now forced to lead as a bug, locked away in his room, causing his family, he realizes, a great deal of shame.
One day, late in the story, he hears his sister playing the violin, and moved by the music and tenderness of his sister´s memory, he goes out of his room to tell her how beautiful the music is that she is playing. This causes an irreversible shock on his sister and family and sets the story forward towards its climax and concluding scenes.