What is an irreversible process of deterioration in the body's systems and organs?

The correct answer is the Cell Death.

The cell death may be the result of the natural process of old cells dying and or might be as the result of different factors like disease or localized injury. The cell death is necessary for the survival of an organism, for its development and growth. There are different types of cell death and different classifications of them. Most commonly it is classified by its s morphological appearance, so it may be apoptotic, necrotic, autophagic or associated with mitosis.