Answer:
The correct answer would be cell.
Cell is the basic structural and functional unit of life or body. For example, nerve cell is the functional and structural unit of nervous system, nephron forms the structural and functional unit of excretory system et cetera.
When many similar cells work together to attain the same function, they form a tissue.
Similarly, similar tissues work in synchronization to form an organ which forms an organ system.
Ultimately, the organ systems form an organism.