It's B. tissue: it contains a group of cells which make up a tissue, which a group of tissues make up an organ, a group of organs make up an organ system and a group of organ systems make up a body.
A group of cells ⇒ A tissue, a group of tissues ⇒ an organ, group of organs ⇒ an organ system, group of organ systems ⇒ a Body
It's not C. Body Cavity: any space in the body
It's not A because, Segmentation is movement of the body with no bones in it.
It's not D because it's the external skeleton that supports and protects an animal's body