The correct answer is: A. Chromatin.
Chromatin is a complex of DNA and proteins found in the nucleus of eukaryotic cells. It is made up of repeating units called nucleosomes, which are composed of DNA wrapped around histone proteins. The nucleosomes are then further organized into higher-order structures called chromatin fibers. These fibers are then condensed into chromosomes, which are visible during cell division.
B. Centromere is a region of a chromosome that is involved in the separation of chromosomes during cell division. It is a constricted region that contains a pair of centromeres, which are necessary for the attachment of the chromosome to the mitotic spindle.
C. Chromosome is a structure in the nucleus of eukaryotic cells that contains DNA. Chromosomes are made up of chromatin, which is a complex of DNA and proteins. Chromosomes are visible during cell division, when they are condensed and become visible as rod-shaped structures.
D. DNA is a molecule that contains the genetic code of an organism. DNA is made up of two strands of nucleotides that are twisted together in a double helix. The nucleotides are arranged in a specific order, which codes for the proteins that are produced by the organism.