The correct answer is D. Heat transfer by conduction through a wall is governed by Fourier's law, which states that the heat transfer rate is directly proportional to the temperature gradient across the wall and the area of the wall, and inversely proportional to the wall thickness. This means that as the temperature difference across the wall increases, the heat transfer rate increases. Similarly, as the wall area increases, the heat transfer rate increases. Conversely, as the wall thickness increases, the heat transfer rate decreases. The other options are incorrect because they do not accurately represent the relationship between the heat transfer rate and the parameters of the wall, as described by Fourier's law.