The children are to do their homework the moment they _______ back school.
A.have arrived
B.had arrived
C.will arrive
D.would arrive
原句有误，似改为：
The children are to do their homework the moment they _______ at school.
名词词组 the moment 可以用作连词，引导时间从句，意思跟as soon as 同。
主句是 be to do 以后时，时间从句用一般目前时。若没，则可以考虑用目前完成时，代替以后完成时，表示以后那个时间业已完成的动作。
故本题选择 A。
语法常识：
目前完成时用在“时间状语从句”或者“条件状语从句”中，表示以后业已完成的动作或状况，代替以后完成时。比如：
1. 时间状语从句：until, as soon as, when
We cant move in until the electricity has been laid on. 大家得等电源接通后才能搬进来。
Ill go to my grannys as soon as I have finished my homework. 我一做完作业就去奶奶家。
Dont forget to switch off when you have finished. 做完之后别忘了关掉。
2. 条件状语从句：if, once, as long as
If you have finished reading the book before I leave, please lend it to me. 假如你在我离开之前就读完了这本书，请借给我。
once we have promised, we should keep it. 一旦许下诺言，大家务必遵守。
You can watch TV as long as you have finished your homework. 只须完成了作业，你就能看电视。
