Answer:
This story is probably about:
B. a brother's relationship with his older sister.
Explanation:
Even without searching for this story, we can make inferences from the excerpt. The narrator is focused on his sister, and the fact that there is something different about her now. Unlike him, she is becoming more mature, which means there is something mysterious about her now. The brother also seems to feel left behind, precisely because of all the mystery surrounding her.
In fact, that is what this story is about. The narrator is Tom, who watches as his sister grows up, which makes them grow apart. She no longer plays with him outside, but practices playing the piano instead. She changes physically and emotionally. Therefore, the best option for this question is letter B. a brother's relationship with his older sister.