During the process of mitosis, spindle fibers play a crucial role in the movement and alignment of chromosomes. These fibers are made of microtubules and are essential for the chromosomes to move accurately into position during cell division. Let's discuss when the spindle fibers attach to the kinetochores of the chromosomes:

• Prophase: This is the initial stage of mitosis, where the chromosomes condense and become visible. However, the spindle fibers have not yet attached to kinetochores. Spindle fibers begin to form, but their attachment to kinetochores happens later.
• Metaphase: This is the stage where the spindle fibers attach to the kinetochores of chromosomes. During metaphase, chromosomes align at the metaphase plate (also known as the equatorial plane of the cell). This alignment is facilitated by the spindle fibers, which pull the chromosomes into position for accurate separation into daughter cells.
• Anaphase: During anaphase, the sister chromatids separate and are pulled apart as the spindle fibers shorten. While spindle fibers are essential during this phase for pulling apart the chromatids, their initial attachment to kinetochores occurs earlier.
• Telophase: This is the final stage of mitosis, where the chromosomes reach the opposite poles of the cell, and the cell starts to re-form its nuclear membranes. Spindle fibers disassemble during this phase.

Based on the description of functions and timing within mitosis, the correct answer for when spindle fibers attach to kinetochores of chromosomes is Option B: Metaphase.