Which of the following best describes a living organism?

The best description of a living organism among the provided choices is that living organisms are physicochemical entities that demonstrate complexity and metabolism. This definition encompasses the critical characteristics that differentiate living organisms from non-living matter.

Living organisms are composed of complex molecules, such as proteins, nucleic acids, carbohydrates, and lipids, which work together to carry out various biological processes. Metabolism refers to the chemical reactions that occur within the organism to maintain life, including processes that convert nutrients into energy (catabolism) and those that build cellular components (anabolism).

In contrast to the other options, this description captures the essential attributes of life, such as the ability to undergo complex biochemical transformations and the organization of biological systems at cellular and molecular levels. It highlights the dynamic nature of living organisms, the interactions between their components, and their adaptation to environmental changes through metabolic processes.