Final answer:
The smallest unit of living matter is the cell, the fundamental building block of life capable of performing all necessary life processes. The cell is the basic independently functioning unit in an organism, according to the unified cell theory.
Step-by-step explanation:
The smallest unit of living matter is the cell. Unlike elements, atoms, or structures like the Golgi apparatus or nucleus, which are either non-living parts or subunits of a cell, the cell is the fundamental building block of life. It is the smallest unit that can perform all life processes, making it the smallest independently functioning biological unit of an organism. Cells provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Furthermore, cells have the ability to reproduce and, in doing so, give rise to new, identical cells. The unified cell theory further clarifies that all living organisms are made up of cells, establishing them as the basic unit of life.
The other options listed, such as the Golgi apparatus and nucleus, are essential components of the cell but do not meet the qualifications of being the smallest unit of living matter on their own. While atoms are the smallest unit of matter, they are not capable of life. Therefore, in the context of biology and according to the unified cell theory, the correct answer is option B) cell.