One of them (a)/ forgot to take their bag (b)/ from the school(c)/ No error (d)

Answer

3

Explanation

Their का जगह his का प्रयोग होगा क्योंकि one of them के लिए possessive Adjective his का प्रयोग होता है न कि their का। one of them का प्रयोग उनमें से एक के अर्थ में होता है । जैसे -
One of them has done his work (true)
One of them has done their/One.s work. (false)