Hey there!
I attached a screenshot of my chemistry notes from this year on equilibrium constants in particular. Feel free to reference this in the future if you wish.
Based on the information in my attachment, you could construct your constant like this:
[tex] K_{eq} = \frac{[CaO][CO_{2}]}{[CaCO_{3}]} [/tex]
I don't think it matters which order your reactants or products are in as long as you have them on the top and bottom where they need to be, so your answer will be B.
Hope this helped you out! :-)
