She is one of the few persons ---- Spanish in our school.B w

首先这是定语从句,先行词,即被修饰名词是人的时候关系词用who.
其次,这道题考查的是先行词中出现one of 的情况.这种情况下,当one of 前没有修饰词时,先行词为one of 后面的名词,如本句的先行词就是persons,因此从句中的动词know不用第三人称单数.而当one of 前有修饰词时,先行词看做单数,即”one“为先行词,如“She is the only one of the few persons"时,从句应该是"who knows Spanish ".