The structures responsible for completing the breakdown of carbohydrates into usable energy for the cell are the A. Nuclei
B. Mitochondria
C. Ribosomes
D. chloroplasts

The correct option is;

B. Mitochondria

Explanation:

The mitochondria are the organelle of the cell bund to the membrane that are referred to as the cell powerhouse they function in the same manner as the digestive system, such that they take in raw nutrients which are broken down to produce energy containing molecules which makes up the majority of the chemical energy required for the various biochemical reactions of the cell in a way such that the energy they produce are stored in adenosine triphosphate (ATP) molecules.

B. Mitochondria

Explanation:

The mitochondria are cellular organelles found in eukaryotic cells. They are structures involved in cellular respiration, which is the process whereby energy in form of ATP is synthesized for use by the cell.

Cellular respiration in eukaryotes occurs in stages including glycolysis, kreb's cycle and oxidative phosphorylation. All these processes combine to breakdown carbohydrates (glucose) into usable energy for the cell. The structures responsible for this process is MITOCHONDRIA.