The difference between tight junctions, desmosomes, and gap junctions is that

Narrow junctions and desmosomes have different structures. Narrow junctions are made of a protein arranged inside the membrane. There are many intersecting lines that cause the cells to be sealed and together. Desmosomes contain the protein cadherin. There are cadherins in each cell and they join at this point sealing the cells together, the tight junctions have the function of preventing the materials that circulate between the cells. The desmosomes provide binding sites for the cells to be attached.

The correct answer is b

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.