The answer is "B. Marsupials". Marsupials undergo relatively premature births compared to many other species, and although this puts the baby at risk, it lessens the risks for the mother.
I think the correct term to fill in the blanks would be glycerol phosphate electron shuttle. This shuttle is responsible of transporting agents that are reducing into the inner membrane of the mitochondria. Since NADH cannot enter, it is reduced so that the electrons could go in to the transport chain.
