Final answer:
During mitosis, sister chromatids separate and become individual chromosomes, occurring in the anaphase stage.
Step-by-step explanation:
When sister chromatids separate during mitosis, they become independent chromosomes, each made of a single DNA strand. The correct answer to the question is (b) Sister chromatids; chromosomes. During the anaphase stage of mitosis, the cohesin proteins degrade, and the sister chromatids separate at the centromere. Each chromatid, now called a chromosome, is then pulled towards opposite poles of the cell.