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]

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

let that person have a turn to speak : )