Final answer:
A chromosome is a structure within the cell nucleus composed of coiled DNA and proteins, responsible for carrying genes.
Step-by-step explanation:
A chromosome is a substance comprised of coiled strands of DNA and special proteins found in a cell's nucleus. This definition confirms that the correct answer to the question is 'a. chromosome'. Chromosomes contain genes, which are units of genetic material that encode for the production of proteins. In human cells, there are typically 46 chromosomes, comprising 22,000 genes. During cell division, the chromatin condenses into chromosomes so that DNA can be safely transported to the daughter cells. The chromosome's structure includes DNA that is wrapped around a set of proteins called histones, forming a nucleosome. These nucleosomes appear like beads on a string where DNA is the string and histones are the beads.