Why is the thermodynamic equilibrium constant, K, unitless?
(a) K is unitless because the units for the concentrations and/or pressures of the products cancel with the concentrations and/or pressures of the reactants.
(b) K is unitless because units are optional in chemistry.
(c) K is unitless because it is a function of activities, which are unitless.
(d) K is unitless because the difference in the concentration and/or pressures cancel out in the equilibrium constant.
(e) None of the above are true, K always has units consistent with the concentrations and/or pressures used.