Explanation:

Mitosis is a process of cell division in which a single cell divides into two identical daughter cells. The process consists of four main phases: Prophase, Metaphase, Anaphase, and Telophase. Each phase has a specific role to play in the process of cell division.

The longest phase of mitosis is Prophase. This is because it is the phase in which the cell prepares for cell division. During Prophase, the cell undergoes a number of changes that are necessary for the separation of the chromosomes.

Some of the changes that occur during Prophase include:

- The chromatin condenses into visible chromosomes.
- The nuclear envelope breaks down.
- The spindle fibers begin to form.
- The centrosomes move to opposite poles of the cell.

All of these changes take time to occur, which is why Prophase is the longest phase of mitosis. Once the cell has completed Prophase, it moves on to Metaphase.

In Metaphase, the chromosomes line up along the equator of the cell, ready for separation. Anaphase follows, during which the sister chromatids are pulled apart by the spindle fibers and move towards opposite poles of the cell. Finally, Telophase occurs, during which the nuclear envelope reforms and the cell begins to divide into two daughter cells.

In summary, Prophase is the longest phase of mitosis because it involves many changes that are necessary for the separation of the chromosomes.