One that is NOT an option that adds $5-7k to the price of the vehicle (as is the case with the GM and Ford trucks). I have not researched cars, but have noticed that the TDI cars from VW seem reasonably priced next to their gasser stablemates.
In all seriousness, if the buyer can't get a diesel up front for a reasonable price compared to the gasoline-fueled alternatives, they have no hope of success.