The basic structural unit of the body is the

Cell

Explanation:

Cell is the basic structural and functional unit of any body. According the the level of organism, these cells come together to form tissues, tissues combine to form organs, organs make organ systems and ultimately a functional body is formed. Without cells an organism would cease to exist. Cells are composed of cell membrane, cytoplasm , nucleus and other cell organelles which help them to coordinate the activities of the organism. According to the organism a cell type could vary. For example plant cells have cell wall while animal cells do not have it.

substitution is the answer in this case

i believe it would be c: tons.