Concept:Mitochondria generate most of the cell's energy as ATP.Explanation:Energy-demanding cells require a continuous supply of ATP.ATP is produced mainly during aerobic respiration inside mitochondria.So, such cells contain more mitochondria to meet their high energy needs.Vacuoles store materials, ribosomes make proteins, and the endoplasmic reticulum transports substances.Hence, the correct option is mitochondria.Answer:B. mitochondria