Biology

The correct answer is: A.Anaphase.

During anaphase, the sister chromatids are separated at the centromere and begin moving toward opposite poles of the cell. Once the centromere splits, each sister chromatid is considered an individual chromosome.

Explanation of the other phases:

B.Prophase: The chromosomes condense and become visible, but the sister chromatids are still attached to each other at the centromere.