Final answer:
The mitotic stage where chromosomes attach to spindle fibers and move towards centrosomes is called prometaphase.
Step-by-step explanation:
The stage of mitosis during which chromosomes begin attaching to spindle fibers and moving towards the centrosomes is known as prometaphase. During prometaphase, the nuclear membrane breaks down, and mitotic spindle fibers extend from the centrosomes (the microtubule-organizing centers in animal cells). These fibers then attach to kinetochores, which are protein structures located at the centromeres of each chromosome. The attachment of spindle fibers to the kinetochores is a definitive characteristic of the prometaphase stage of mitosis.