The primary effect of aging on all body systems is a. senility. b. programmed senescence. c. decreased functional reserve. d. diseased function.

C) decreased functional reserve

Explanation:

Ageing is a biological process which leads to the loss of function of the tissue and cells of the organism.

The biological ageing is determined genetically and can be correlated with the chronological ageing. The ageing is also influenced by the environmental factors which could interfere with the physiology, anatomy and morphology of the organisms.

The cell division cycle gets arrested to prevent the proliferation of these damaged cells and to remove the harmful factors from the body. This leads to the loss of function of the cells and tissues.

Thus, Option-C is the correct answer.