_____ is the fundamental structural unit of living organisms.

Cell

The correct answer is Option (4)- Cell

The fundamental structural unit of living organisms is the cell. Cells are the basic building blocks of life, responsible for the structure, function, and organization of all living organisms. They vary in size, shape, and function but share common features such as a cell membrane, cytoplasm, and genetic material (DNA). Cells can exist as independent entities or form tissues, organs, and organ systems through specialization and cooperation. Thus, while organs, tissues, and organ systems are important components of living organisms, cells are the fundamental unit upon which all life is based.