A branch of the dominant coronary artery that supplies the inferior portion of the septum. What is the single most appropriate option?

The key is D. Left main stem, post descending artery. [Here no option is satisfactorily correct! By dominant coronary artery we mean that coronary artery which gives of the branch of posterior descending artery. Mostly it is right coronary artery and if there is left coronary dominance, posterior descending artery is the branch of circumflex artery and not direct branch of left main artery. However the only option that goes nearer is D. Left main stem, post. descending artery].