The correct answer is C. for.
The preposition “for” is used to indicate the purpose or intention of something. In this case, the speaker is asking if the listener is ready to eat dinner. Therefore, the correct answer is “for dinner.”
The other options are incorrect because they do not make sense in the context of the question. Option A, “of dinner,” is incorrect because it does not indicate the purpose or intention of something. Option B, “on dinner,” is incorrect because it does not make sense to say that someone is “ready on dinner.” Option D, “off dinner,” is incorrect because it means the opposite of what the speaker is asking.
In conclusion, the correct answer to the question “Are you ready for dinner?” is C. for.