Secretions from the liver and the pancreas get mixed with food here.  by the time food leaves this organ, the process of digestion is complete. which organ are we talking about?

only b 

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.