D. S phase of Interphase.
The 2nd chromatid is made during DNA replication, where each linear DNA molecule is replicated to result in genetically identical pairs of DNA strands.
During Prophase stage of Mitosis, each strand of DNA condenses to form the sister chromatid of each metaphase chromosome. Sister chromatids are linked to each other via kinetochore proteins linked to the centromeric regions of each sister chromatid.
Thus, sister chromatids are genetically identical to each other.
Hope this helps! :)
