I put a request for help on the Military Pilots forum
here.
The two problems we have encountered are that the RAF navigators used to work out minimum time routing by flying pressure levels, which we don't do, and they also back plotted the route from destination. The technique we are describing seems to be a simplified version and it's difficult to get an answer other than by applying logic, as moggie has done. The consensus seems to be answer (d).
I think this question originated in France. Ian Seager, from Flyer, is going to make some enquiries there. If we get anything new I'll get back to you.