The correct option is D (mol/L)12
Equilibrium constant Kc is proportional to the active masses of the products and reactants.
C(s)+12O2(g)→CO(g)
For this reaction, Kc=[CO][O2]12
Active mass of solids are considered as 1.
where [CO], [O2]12 are the molar concentations having a unit of mol/L
So, unit of Kc=mol/L(mol/L)12
Kc=(mol/L)12