Desmatosuchus was a crocodile relative that lived only during the Triassic Period. Horsetails are
plants that first appeared in the Triassic Period and still exist. Which of these organisms would be more
useful as an index fossil for the Triassic Period? Why?

The correct answer is c.

Im assuming this is a multiple choice answer what is the answers to chooce from?

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.