Correct Answer: C

Rationale: The cell is the smallest structural and functional unit of life, serving as the basic building block for all living organisms. Cells carry out essential processes such as energy production, reproduction, and response to stimuli. The cell theory, developed by Schleiden and Schwann in the 19th century, states that all living things are composed of one or more cells and that all cells arise from pre-existing cells. Cells vary in complexity from simple prokaryotic cells (like bacteria) to highly organized eukaryotic cells found in plants and animals. Their study, known as cytology, is foundational to modern biology.

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