Concept:The expression “no option but to” is used to show that only one choice remains.
Explanation:The sentence means that if the student’s grades do not improve, the speaker will have only one possible course of action.
That single action is moving the student to another school.
In standard English, the fixed phrase is “have no option but to” followed by a verb.
Therefore, the correct word is “but”.
Option A, “than”, is not used after “no option”.
Option B, “except”, and Option C, “besides” also do not form the correct idiomatic expression.
Only “but” naturally completes the sentence: “no option but to move you to another school.”
Answer:D. but