What are threadlike structures made of DNA molecules that contain genes called?

The correct choice identifies the threadlike structures that are composed of DNA and serve as the repositories for genes as chromosomes. Chromosomes are found in the nucleus of eukaryotic cells and are crucial for the organization and regulation of genetic material during cell division. Each chromosome contains a single molecule of DNA that is tightly coiled and packaged with proteins.

Nucleotides, while they are the fundamental building blocks of DNA and RNA, represent the individual units that make up the strands of genetic material but do not describe the larger structures that contain genes. Centromeres are specific regions on chromosomes that play a role in the separation of chromosomes during cell division but do not represent the entire structure containing genes. Histones are proteins that help package DNA into a compact form, allowing it to fit within the nucleus, but they are not the structures themselves that contain the genes. Thus, chromosomes are the correct term for the structures made of DNA molecules that contain genes.