Give some stream of consciousness and then the answer. Cell nuclei contain most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are the cell's nuclear genome and are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression -- the nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.

Answer this question: why is the nuclues called the control center of the cell?
The relevant information is: The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression -- the nucleus is, therefore, the control center of the cell.
Therefore, the answer is The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression.