Is Vonnegut's story finally a cautionary tale about the importance of freedom? Of individuality? Of human excellence? Or is he aiming at something else?

Yes, It is a cautionary tale about the importance of freedom and equality, of individuality and excellence.

Option A.

Explanation:

Harrison Bergeron is a cautionary tale written by Kurt Vonnegut. In that tale, he had written about the ideas of freedom and equality and how to be able to express their thoughts and ideas and being an individual. In that tale, he is thriving and striving for equality.

This tale is about ending any kind of discrimination on any of the grounds like caste, creed, color, religion and so on and striving towards having equality and freedom.