Final answer:
The life span of an organism refers to the time it takes for the organism to complete its life cycle, from birth to death.
Step-by-step explanation:
The correct definition of 'life span' is the time it takes for an organism to complete its life cycle. This includes the period of gestation, birth, maturity, senescence, and death. The life span of different organisms can vary greatly, from a few days for certain insects to several decades or even centuries for certain animals.