What structure contains the DNA and RNA of a cell?

The nucleoid is the correct answer because it serves as the region in prokaryotic cells where the cell's genetic material, DNA, is located. In prokaryotes, such as bacteria, DNA is not contained within a membrane-bound nucleus. Instead, it exists in a concentrated area known as the nucleoid, which holds the essential genetic information required for the cell's functions and replication.

In addition to DNA, RNA is also present within the nucleoid because it is synthesized from DNA during the process of transcription, essential for protein synthesis and other cellular processes. This central role of the nucleoid in housing the cell's genetic material makes it the key structure for genetic regulation and expression.

Other structures mentioned, like the cytoplasm, do contain various cellular components, including ribosomes and nucleic acids in a dispersed form, but they do not specifically house the DNA in the organized manner that the nucleoid does. The cell wall is primarily a structural component providing shape and protection to the cell and does not contain genetic material. Similarly, a capsule serves as a protective layer for some bacteria but is not involved in storing DNA or RNA.