During which phase of mitosis do the sister chromatids line up in the center of the cell?

The correct answer will be metaphase.

Explanation:

Mitosis is a type of cell division in which parent cell divides to form two daughter cells with same genetic material as of parent cell.

Mitosis proceeds into 4 phases: prophase, metaphase, anaphase and telophase.

Metaphase is the phase of cell division marked by the alignment of the chromatids at the equatorial line or in the center of the cell and condensation of the chromatids to form chromosomes which can be easily observed under microscope.

Thus, metaphase is the correct answer.

They line up at Metaphase