The correct answer is D. of.
The sentence “I don’t approve of your language” is an expression of disapproval. The speaker is saying that they do not like the way the listener is speaking. The word “of” is used to indicate the object of the disapproval. In this case, the object of the disapproval is the listener’s language.
The other options are not correct because they do not make sense in the context of the sentence. The word “from” is used to indicate a source. In this case, it would not make sense to say that the speaker does not approve from the listener’s language. The word “in” is used to indicate a location. In this case, it would not make sense to say that the speaker does not approve in the listener’s language. The word “on” is used to indicate a surface. In this case, it would not make sense to say that the speaker does not approve on the listener’s language.