Under a light microscope, a stained cell shows chromosomes lined up along the middle. what will happen next?

When the chromosomes lines up in the middle it is called metaphase. after that the chromosomes divide to form sister chromatids then it moves to the opposite poles.

Separation and movement of sister chromatids to the opposite poles as a result of shortening of the spindle fiber.

Explanation:

Chromosomes lining up along the middle of the cell is one of the stages of mitotic cell division known as metaphase. Generally, mitotic cell division consists of 4 stages;

The prophase stage during which the nuclear envelope dissolves, the chromatin material condenses and become distinctly viewed as chromosomes. In addition, spindle formation becomes initiated at the two ends of the cell and grow towards the center.The metaphase stage is marked by the alignment of the chromosomes at the middle of the cell also known as the equator. Thereafter, spindles from the opposing cell ends engage sister chromatids at the centromere.The anaphase stage is marked by shortening of the spindles which leads to the separation of sister chromatids of the same chromosome towards opposing ends of the cell.Telophase stage is the stage at which chromatids complete their migration and reach the opposing ends.

Since chromosomes aligning at the middle is corresponds to the metaphase stage, the next stage is anaphase. A stage during which sister chromatids separate and starts migrating towards the opposite poles as a result of shortening of the spindle fibers.

The answer is option a.