The correct option is C. Cell. Explanation of Why the Answer is Correct:

1. Definition of a Cell: A cell is the fundamental unit of life. It is the smallest structure that can carry out all the processes necessary for life. Cells can exist independently as single-celled organisms (like bacteria and protozoa) or as part of multicellular organisms (like plants and animals).
2. Single-Celled Organisms: Organisms such as bacteria, archaea, and some protists are made up of just one cell. These organisms perform all life functions within that single cell, including metabolism, reproduction, and response to the environment.
3. Multicellular Organisms: In multicellular organisms, cells specialize and work together to form tissues, organs, and systems. For example, in humans, muscle cells form muscle tissue, which combines to create organs like the heart.
4. Cell Theory: The concept of the cell as the basic unit of life is supported by cell theory, which states:
5. All living organisms are composed of one or more cells.
6. The cell is the basic unit of life.
7. All cells arise from pre-existing cells.

Explanation of Why the Other Options are Wrong:

• A. Tissue: A tissue is a group of similar cells that work together to perform a specific function. Tissues are made up of cells, but they cannot exist independently as single-celled organisms. Therefore, tissue is not the basic unit of life.
• B. Organelle: Organelles are specialized structures within a cell that perform specific functions (like mitochondria for energy production or ribosomes for protein synthesis). While organelles are essential for the cell's function, they are not capable of existing independently and do not represent the basic unit of life.
• D. Organ: An organ is a structure composed of different types of tissues that work together to perform a specific function (like the heart or lungs). Organs are part of multicellular organisms and cannot exist as single-celled organisms. Thus, they are not the basic unit of life.

Summary of Key Points:

• The cell is the basic unit of life, capable of existing as a single-celled organism or as part of a multicellular organism.
• Single-celled organisms perform all life functions within one cell, while multicellular organisms have specialized cells that form tissues and organs.
• Cell theory supports the idea that all living organisms are made of cells, which are the fundamental units of life.
• Other options (tissue, organelle, organ) do not represent the basic unit of life as they are either composed of cells or cannot function independently.

This understanding of cells is crucial for grasping more complex biological concepts and processes, making it a foundational topic in biology.