1.-Hey, you haven't been acting like yourself. Everything OK ?
A. I'm fine, thanks B. Sure, it is C. That's good D. It's OK
The given answer is A.
How would native speakers feel about and respond to the test question? Thank you in advance.
As far as I'm concerned, possible responses to what is a fairly informal question are A, B (without the comma) and D. C is the only one that doesn't fit at all.