The correct answer is C) temperature is equal to, or below the dew point temperature. Fog forms when the air temperature is equal to or below the dew point temperature, which is the temperature at which water vapor in the air condenses into tiny water droplets. When the air temperature reaches or falls below the dew point, the air becomes saturated with water vapor, and the water vapor condenses to form fog. The other options are incorrect because: A) Higher air temperature compared to the dew point would not lead to fog formation. B) 90% water saturation is not required for fog formation. D) 50% water saturation is also not the specific condition for fog formation.