Nucleolus and endoplasmic reticulum start disappearing at

The correct option is early prophase.

To understand the process of cell division, let us break down the key events that occur during prophase, the first stage of mitosis:

1. Early Prophase:
- The chromatin fibers condense to form distinct, visible chromosomes.
- The centrosomes, which duplicated during the S phase, begin to move toward the opposite poles of the cell.
- Important cellular structures, including the nucleolus, the endoplasmic reticulum (ER), and the Golgi complex, begin to disintegrate and start disappearing from view.

2. Late Prophase:
- By the end of prophase, the nuclear envelope completely breaks down.
- The nucleolus and endoplasmic reticulum are no longer visible under the microscope, allowing the chromosomes to be released into the cytoplasm where spindle fibers attach to them.

Since the question asks when these structures start disappearing, the process begins in early prophase.