Step 1: Understand the sentence.
The sentence describes a teacher giving two instructions: first, Kamal was asked to leave the room, and second, something was done regarding his return. The blank should logically express prohibition.
Step 2: Analyze the options.
Stopped means prevented, but it does not fit grammatically with "him to return".
Refused means declined to do something, which does not fit the context here.
Forbade means not allowing someone to do something. The structure "forbade him to return" fits both grammatically and logically.
Challenged means dared or invited someone to compete, which is irrelevant here.
Step 3: Conclusion.
Thus, the correct word is forbade, as the teacher did not allow Kamal to return.