Answer:
C. The author is impressed by her friend.
Explanation:
According to the given passage, the author details how busy her friend Sadia is. Not only is she active in sports, but she also ills her spare time "tak[ing] classes at the community college."
While clearly impressed by the friend, the author couldn't help but add some facts of her own, just to emphasize the busy schedule of Sadia. She added that she thinks "she composes symphonies on the weekends and probably dabbles in particle physics too", though most probably "for laughs". This phrase shows that this opinion is nothing but her own.
Thus, the correct answer is option C.