A0 are the smallest biological structural units of the body.

The correct answer for the blank is-

Cell.

Cell is the smallest biological, structural, and functional unit of all life forms ( such as plants, animals, and bacteria). In other words it is the basic unit of all life forms as all the living organisms are composed of cell.

Cell can exist independently and performs all the cellular processes ( metabolic processes)  that are necessary for the survival.

Thus, cell is the right answer.

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.