majors (on witch) are not "hot" today may become the "hot" m

楼主是不是看错答案了?
答案用which或者that就可以了.
翻译：今天（现在）并不热门的专业,可能会变成明天（将来）的热门专业.
主句是：Majors may become the 'hot' majors of tomorrow.
而___ are not "hot" today为定语从句,修饰主句主语majors.
将先行词majors代入该定语从句,很明显它可以直接做从句的主语.
所以直接用which或者that就可以了,而不能使用on which.
楼主可能是看错答案了,或者答案自身有误,或者答案印刷错误.
总之对自己要有信心.