Explanation:
## Step 1: Identify the Basic Unit of Life
### The question asks for the fundamental unit that makes up all living organisms. In biology, it is well-established that the cell is the basic unit of life. All living organisms are composed of cells, which perform essential functions necessary for life.
## Step 2: Evaluate Each Option
- **A. hệ cơ quan (organ system)**: An organ system is a group of organs working together to perform major functions or meet physiological needs of the body. It is not the basic unit of life.
- **B. cơ quan (organ)**: An organ is a part of an organism that is typically self-contained and has a specific vital function. Examples include the heart, liver, and lungs. While important, it is not the most basic unit.
- **C. tê bào (cell)**: The cell is the smallest unit of life that can replicate independently, and cells are the building blocks of all living things. This is the correct answer.
- **D. mô (tissue)**: Tissue is a group of similar cells that work together to perform a specific function. Tissues combine to form organs, but they are not the most basic unit of life.
## Step 3: Confirm the Correct Answer
### Based on the evaluation, the correct answer is "C. tê bào (cell)" because it is the fundamental unit of structure and function in all living organisms.
# Answer:
### C. tê bào
