In the equation : ΔG = -RTlnK (i.e at equilibrium), is K = K_c or K_p ?

At equilibrium,
Delta G = – RT ln Kc
where,
Delta G = Change in Gibb's free energy
R = Gas constant
T = Temperature and,
Kc= Equilibrium constant which is defined as the ratio of product of the equilibrium concentrations of the products to that of the reactants with each concentration term raised to the power equal to the stoichiometric co-efficient of the substance in the balanced chemical equation