What is the dense, active protoplasm found in the center of a cell called?

The dense, active protoplasm found in the center of a cell is known as the nucleus. The nucleus serves as the control center of the cell, housing the cell's genetic material (DNA) and coordinating various cellular activities, including growth, metabolism, and reproduction. It is often described as the "brain" of the cell because it regulates gene expression and mediates the replication of DNA during the cell cycle.

In the context of the other options, the cell membrane acts as a protective barrier that encloses the cell but does not contain the dense protoplasm at the center. Cytoplasm refers to the jelly-like substance that fills the cell and surrounds the nucleus, but it is not contained in the nucleus itself. The endoplasmic reticulum is an organelle involved in protein and lipid synthesis located within the cytoplasm, further distinguishing its function from that of the nucleus.