Organisms need a way of storing energy because
a. a cell can't always immediately use all the energy it gets
b. an organism often has times when no energy is used
c. a cell can release only stored energy
d. a cell cannot create energy and must get it from elsewhere in the organism

The correct answer is A. a cell can't always immediately use all the energy it gets. That's why the organism stores energy for future use. Fat on a body is essentially that storage of energy and if the organism would be exposed to starvation, the body would start losing fat in order to use the energy. That's why bears and other hibernating animals eat so much and are so fat. That's because they need the fat layers full of energy to help them through the winter.