Please, don't report the bug here, but in the report window, under the exercise page. The course creator will see it if you send it with the report window, but not here.
Anyway, maybe some Hindi speaker people use this app to learn English, and could take the correction as proper English.
Aur. Because the "o" and the "au" are different.
"o" is closed o:
"au" is "ɔ" in IPA, open o:
Most native Hindi speakers that I speak with use पुस्तक(Pustak) instead of किताब (Kitab) for book. But I see किताब used more in Bollywood movies.
Is it ok to use पुस्तकinstead? From what I understand किताब is an Arabic word, and is falling out of style. At Least that's what it seemed like when I spoke with native Hindi speakers.