Final answer:
Chromosomes are duplicated during the S phase of the cell cycle, resulting in the formation of sister chromatids held together at the centromere.b) S phase
Step-by-step explanation:
Chromosomes are duplicated during the S phase of the cell cycle. In this phase, DNA replication occurs, resulting in the formation of two identical copies of each chromosome called sister chromatids. These sister chromatids are held together at the centromere.