They want to be [ ] A teather Ba teather Cteathers，，

You'd choose B as the proper answer.

This sentence is expressing their wish of being a teacher, that is, "They" want to seek "teacher" as their profession, so either A or C is not proper for this sentence. It is an expression of a common sense.
And, it is different to another sentence: "I am a teacher, we are teachers." Just because in this sentence, the "teacher" serves as "our" identifications.