the smallest unit of life is
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tissue
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cell
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organ
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organ system
The cell is the fundamental unit of life. All living organisms are composed of cells, which are the smallest structures capable of performing all life processes. Tissues are groups of cells, organs are groups of tissues, and organ systems are groups of organs - all built upon the cellular foundation.
The cell is universally recognized in biology as the smallest structural and functional unit of life — every living organism is composed of one or more cells, and the cell is the smallest unit capable of independently carrying out all the processes of life (metabolism, growth, reproduction, response to stimuli) when isolated. This is foundational cell theory, taught from middle-school biology onward. An 'organ' (e.g., heart, liver) is a much larger, more complex structure made of multiple tissues, which are themselves made of many cells — organs sit far above the cellular level in the hierarchy of biological organization (cell → tissue → organ → organ system → organism). The DB's marked answer, 'organ,' inverts this hierarchy and is factually wrong; 'cell' was offered as an option but incorrectly marked false.