Is this school --- you visited last year?A.the one B.that C.

变成陈述句就简单了：
This school is ____ you/I visited last year.
如果直接选关系词引导定语从句的话,则主句为This school is.
主句都不完整,这是不符合英语语法的.
只有选A,先给主句加上表语及给定语从句加上先行词才可以.
This school is the one (which/that) you/I visited last year.
因为关系代词which/that代替the one即the school,在定语从句中做visited的宾语,所以which/that可以省略不写.
所以答案A为正解.