According to me, I think this chord is A°/C, with the D as an accented passing note. Is this correct?
Ok, the comments has been moved to chat with the argument that an answer should be posted as an answer not a comment. That makes sense.
So there should be an actual answer and I will post the answer here as it appeared in the comments:
Yes it is correct that the chord is A°/C, with the D as an accented passing note.
An A diminished in first inversion (over C) - the D is an accented (on a down beat) passing note.