What is the fourth step in mitosis where non-kinetochore spindle fibres lengthen and elongate the cell?

The correct option is Anaphase.

Mitosis is a highly organized cell division process divided into several sequential stages that ensure the duplicate genetic material is distributed equally. These stages include:
1. Prophase: Chromatin condenses into distinct chromosomes, and the mitotic spindle begins to form.
2. Prometaphase: The nuclear envelope breaks down, and microtubules attach to chromosome kinetochores.
3. Metaphase: Chromosomes align along the equatorial metaphase plate.
4. Anaphase: Sister chromatids are separated and pulled toward opposite poles of the cell, and the cell undergoes elongation.
5. Telophase: Nuclear envelopes reform around the two sets of chromosomes, which begin to decondense.

During anaphase, two main mechanical actions occur simultaneously:
- Shortening of kinetochore microtubules: This action pulls the separated sister chromatids toward the opposite spindle poles.
- Lengthening of non-kinetochore microtubules: These spindle fibers (also called polar microtubules) do not attach to chromosomes. Instead, they overlap at the center of the cell and slide past one another using motor proteins. As they polymerize (lengthen) and slide, they push the two poles of the cell further apart, effectively elongating the entire cell to prepare it for cytokinesis (physical division of the cytoplasm).