The appropriate name of the chromosome or structure labeled A-D is as follows:
A - chromosome
B - Centromere
C - chromatid
A chromosome is a linear arrangement of condensed DNA and associated proteins (such as chaperone proteins) which contains the genetic material (genome) of an organism.
A centromere is the central region of a eukaryotic chromosome where the kinetochore is assembled. The two “sister” chromatids are joined at a constricted region of the chromosome called the centromere.
A chromatid is either of the two connected double-helix strands of a metaphase chromosome that separate during mitosis.