Final answer:
The stage of mitosis described, where chromatin condenses, spindle fibers form, and centrioles organize spindle fibers, is prophase.
Step-by-step explanation:
The stage of mitosis being described is prophase. During prophase, chromatin condenses to form visible chromosomes. In animal cells, centrioles begin to move towards opposite poles of the cell, and the spindle fibers, made of microtubules, start to form a structure known as the mitotic spindle. This spindle consists of fibers that attach to the chromosomes and will later assist in their separation during subsequent stages of mitosis. Additionally, during prophase, the nuclear envelope breaks down to allow these interactions between chromosomes and spindle fibers.