Under which atmospheric conditions is a fumigating plume from a smoke stack most likely to form?

a) There is a ground-based temperature inversion. Above the inversion, the temperature decreases with elevation at nearly the same rate as the adiabatic lapse rate.
b) The stack is pretty high off the ground, way above the top of the temperature inversion.
c) There is a ground-based temperature inversion with an unstable atmosphere above the inversion. The stack height is in about the middle of the temperature inversion.
d) There is a ground-based temperature inversion with an unstable atmosphere above the inversion. The stack height is just below the top of the inversion.