The correct answer is D) fall, around early morning. Steam fog is most likely to occur on the Mississippi and Ohio Rivers in the fall, around early morning. This is because during the fall season, the air temperature drops and the water temperature remains relatively warm, leading to the formation of steam fog over the river. The early morning hours are when the temperature differential between the air and water is greatest, making it the prime time for steam fog to develop. The other options are incorrect because spring does not typically experience the same temperature differential as fall, and midday or late evening are not the prime times for steam fog formation on these rivers.