The organelle primarily responsible for moving the sister chromatids within the cell is the mitotic spindle, which is composed of microtubules organized by the centrosomes.
In the context of your options:
- Metaphase: Sister chromatids are aligned at the metaphase plate.
- Anaphase: The mitotic spindle shortens, pulling the sister chromatids apart towards opposite poles of the cell.
- Prophase: Chromosomes condense and the mitotic spindle begins to form.
- Telophase and cytokinesis: Two new nuclei form, and the cell begins to divide.
From these options, anaphase is the stage where the movement of sister chromatids occurs.