Consider the following reversible reaction

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]

not clear

because of all the things that happen around the us, many people get into arguments which sadly could get physical, but we should actually make a change for that.