Unless he apologizes (1)/ he should not be (2)/ allowed to stay with us. (3)/ No error (4)

Answer

2

Explanation

will/can will replace should because - when we use Simple Present Tense in the Condi tional Clause, will/can/may will be used in the Main Clause.
Look at the examples given below: Unless you study diligently, you.ll never understand trigonometry. Unless we hurty, we will be late for the show. Hence, he will not be is the right usage