The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology.

What are the smallest unit of life called? Okie... think carefully first, then make a decision:
To answer this question, we should know that: A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". So, the answer is A cell.