BFinal answer:
The correct sentence supporting the inference about Alexandra's attitude change is option C, which indicates that her disposition is affected by her friends' opinions.
Step-by-step explanation:
The sentence that best supports the inference that Alexandra's attitude changes depending on the people she is spending time with is: "Without the thoughts of her friends disapproving of this "babyish" activity, Alexandra really allowed herself to have fun."
This sentence clearly shows that Alexandra's enjoyment and attitude are influenced by the people around her, particularly her friends, whose disapproval seems to affect her willingness to enjoy activities that they would consider uncool or immature.
Therefore, the correct option is C.