Answer: The part of the cell theory that best explains why unicellular organisms can perform all life's basic functions is "all living things are composed of cells."
Explanation: This means that unicellular organisms are composed of a single cell that has all the necessary components and functions to carry out life processes on its own. The cell is the basic unit of life and contains all the necessary structures, such as a cell membrane, cytoplasm, and genetic material, that allow it to carry out functions such as reproduction, metabolism, and response to stimuli. Therefore, unicellular organisms can perform all basic life functions because their single cell contains all the necessary components to do so.