In what structure is DNA found in eukaryotic cells?

In eukaryotic cells, DNA is primarily found in the nucleus. The nucleus serves as the command center of the cell, housing the cell's genetic material. This structure not only contains the DNA which is organized into chromosomes but also regulates gene expression and DNA replication, ensuring proper functioning and heredity of genetic information.

The nucleus is surrounded by a double membrane called the nuclear envelope, which protects the DNA and controls the exchange of materials in and out of the nucleus. Other cell structures such as the cytoplasm, ribosomes, and endoplasmic reticulum do not contain the DNA; rather, they play different roles in cellular processes. The cytoplasm provides a medium for chemical reactions, ribosomes are involved in protein synthesis, and the endoplasmic reticulum assists in the synthesis and transport of proteins and lipids. Thus, the nucleus is the correct answer as the primary repository of DNA in eukaryotic cells.