What organelle reforms around the chromatids at opposite ends of the cell?

Nucleus

Explanation:

Chromatids are the linearly arranged condensed structure of genetic material in the cell. The chromatids are observed after the parent cell are divided into the daughter cells.

During cell division, the cell divides, their chromosomes separate and gets accumulated at the poles of the cell. These chromatids are naked that is floating in the cytosol but the characteristics feature of the eukaryotic cell is the presence of a double membrane organelle which can protect the genetic material called the nucleus.

Thus, nuclear envelope or nucleus gets reformed around the chromatids.