The cell structure made up of dense protoplasm that is found in the center of each cell and plays an important role in cell reproduction is?

The nucleus is the correct answer as it serves as the control center of the cell, housing the cell's genetic material (DNA) and coordinating activities such as growth, metabolism, and reproduction. During cell reproduction, the nucleus is crucial because it ensures that the genetic information is accurately copied and divided between the daughter cells. This process is essential for maintaining genetic continuity across generations of cells.

While the cytoplasm, ribosome, and mitochondria each play vital roles in cellular function, they do not specifically focus on the storage of genetic material or the process of reproduction like the nucleus does. The cytoplasm is the jelly-like substance that fills the cell and allows for the movement of materials, ribosomes are responsible for protein synthesis, and mitochondria are known as the powerhouses of the cell, producing energy. Each of these organelles contributes to the cell's overall operations, but the nucleus is uniquely positioned to oversee the complex process of cell reproduction.