Choose the correct form of adjective for the given sentence.
Knowing no better, he used very ______ language.

Here in the sentence, we need an adjective to qualify the noun language.
Now let's understand the meaning of other options:
Inappropriate: Not suitable for a particular occasion etc
Spontaneous: Happening or arising without apparent external cause
Accurate: Conforming exactly or almost exactly to fact or to a standard or performing with total accuracy
Correct: Socially appropriate or approved