The correct option that completes the above statement is C.
he chose to stay at the new school when his brother left.
This makes more sense because the use of the conjunction, "but" should introduce a contrasting idea after the main clause.
Thus, after stating that the "narrator transferred to the new school with his brother," the idea should be completed with "but he (the narrator) chose to stay at the new school when his brother left."
Read more about the use of conjunctions at https://brainly.com/question/14616129