Answer:
The correct option is B. Nucleus
Step-by-step explanation:
The structure in the diagram is a nucleus because the nucleus is a membrane-bound organelle mostly round in shape and it is present inside the cells.
The structure in the diagram cannot be cytoplasm as cytoplasm is a fluid in which organelles float.
The structure in the diagram cannot be a cell wall as a cell wall is the outer layer of a cell and it is not present inside of the cell.
The structure cannot be a flagellum as flagellum are hair-like structures. Hence, option A, C and D are false.