when定语从句。把句子分解就是。The students will put off the outing until next week. They won;t be so busy at that time/ next week.很显然在第二句中: at that time/ next week为时间状语,故用when引导。
答案A when翻译:学生们把出游推迟到下周,这是他们不太忙的时候。特殊疑问词when 引导的定语从句。