Final answer:
The correct answer is A. a cell, as it is the smallest independently functioning unit of a living organism, and the fundamental building block for all life forms according to cell theory.
Step-by-step explanation:
Based on cell theory, the lowest level of organization is a cell (A), which is the smallest independently functioning unit of a living organism. Organelles (B) like mitochondria and chloroplasts are components within cells that have specific functions but are not independent living entities by themselves. Tissues (C) are groups of similar cells that work together to perform a specific function, but they are not the lowest level of organization. Finally, cellular molecules (D) such as proteins and nucleic acids are essential for the functioning of cells but do not have the complexity required for life on their own. Therefore, the cell is fundamental as all living organisms, ranging from simple bacteria to complex multicellular organisms like humans, are made up of cells.