Spindle fibers attach to kinetochores of chromosomes during

The correct option is Metaphase.

To understand the process of chromosome segregation, let us review the key stages of mitosis and identify where spindle fiber attachment occurs:

1. Prophase: The chromosomes condense and become visible under a microscope. The mitotic spindle begins to form outside the nucleus, and the centrosomes move toward opposite poles. At this stage, the nuclear envelope is still intact or just beginning to break down, so spindle fibers cannot yet access the chromosomes.
2. Metaphase (including Prometaphase): As the nuclear envelope breaks down completely, the spindle fibers (microtubules) invade the nuclear region. These fibers attach to the kinetochores, which are specialized protein complexes located at the centromere of each chromosome. The pulling and pushing forces exerted by these microtubules align the chromosomes along the cell equator (known as the metaphase plate).
3. Anaphase: Once all chromosomes are properly aligned and attached during metaphase, the sister chromatids are separated and pulled toward opposite poles of the cell.
4. Telophase: The chromosomes arrive at opposite poles, decondense back into chromatin, and new nuclear envelopes reform around them.

Therefore, the attachment of spindle fibers to the kinetochores is a critical prerequisite for aligning the chromosomes, which is the defining characteristic of Metaphase.