organic chemistry - What is the hybridization of the carbonyl oxygen in a carboxylic acid?

My professor sent out a review guide. One of the questions on this guide is

The carboxylic acid carbonyl oxygen is:

1. $\ce{sp^2}$
2. $\ce{sp^3}$
3. $\ce{H}$-bond donor
4. rapidly equilibrating between $\ce{sp^3}$ and $\ce{sp^2}$
5. 1 and 3
6. 1 and 4

I chose (4) because of the resonance that can be drawn with a negative oxygen and positive carbon. However apparently the answer is (6). I don't understand how something can be equilibrating between two states and still be one of those states.

Could anyone explain?