I knew Alice’s watch.It must be ----(she).这题我做错了,那个大侠帮帮me

该用传说中的“名词性物主代词”——hers.
名词性物主代词如mine,hers,yours,应这样用：
The shirt is mine.
The pencil is yours.
形容词性物主代词如my,her,his,应这样用：
This is my shirt.
This is your pencil.