The table below shows the fossils found in various layers of an undisturbed rock:
fossils in rock layers
layer (from top) fossil in layer
1 a, c
2 e
3 a, d, f
4 b
5 f, c
6 d, b
which is most likely an index fossil?
a
b
d
e

It would be 232.4242

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.