Scenario B would be the most costly and would have the highest heat load. The reason? Air leakage. Air infiltration is a large component of the heat load on a house and the colder the air that's leaking in, the greater the load, everything else being equal.

If there was zero air leakage (practically impossible) scenario C would be the most costly since the radiant loss of the structure (which is proportional to the forth power of absolute temperature) would be slightly higher than the other two scenarios.

Paul in Montreal.

Here is a link that might be useful: Stefan-Boltzmann Law

Disagree, they are all about the same. The argument of leakage is incorrect since if you assume the same infiltration, you heat it up to the same differntial (40 degrees in each case).
Also, the radiation loss has an effective h of about unity at these temps and so it would also lead to a negligible difference in each case.The radiation loss, by the way, is proportional to the difference in the 4th powers of outside and outside wall absolute temperatures, not as stated. Overall, the difference in the three cases is negligible.