How would you express the solubility product (K_sp) of a compound using activity coefficients?

K_sp = c(concentration of C) • a(concentration of A) • b(concentration of B)

K_sp = (activity of C)^c(activity of A)^a(activity of B)^b

K_sp = [C]^c[A]^a[B]^b

K_sp = (activity coefficient of C)^c(activity coefficient of A)^a(activity coefficient of B)^b