Consider the following reversible reaction

Answer:
The right choice is K(eq) = ([CO₂]) / [O₂]
Explanation:
The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium
For a system undergoing a reversible reaction described by the general chemical equation
An equilibrium constant can be expressed as following
K(eq) = ([X]ˣ*[N]ⁿ) / ([A]ᵃ*[B]ᵇ)
where [] is the equilibrium concentration of each component.
and the concentration of solid is conventionally considered as 1
in your example:
The reversible reaction
taking in consideration that C is a solid and the concentration of solid is conventionally considered as 1
So, the equilibrium constant for this reversible reaction can be expressed as following
∴ The right choice is K(eq) = ([CO₂]) / [O₂]