Final answer:
The mitotic phase where chromatin condenses and spindle fibers form is Prophase. During this phase, chromatin turns into visible chromosomes, the nuclear envelope breaks down, and the spindle apparatus is formed to later separate the chromosomes.
Step-by-step explanation:
The mitotic phase during which chromatin condenses and spindle fibers begin to form is called Prophase (option A). During this phase, the DNA which is in the form of chromatin, becomes tightly wound into chromosomes that are visible under a microscope. As the centrioles move to opposite poles of the cell, a spindle composed of microtubules starts to form. Additionally, the nuclear envelope breaks down during prophase, which allows the spindle fibers to access the chromosomes.
Prophase can be further divided into sub-phases, including prometaphase where spindles attach to kinetochores on centromeres of sister chromatids. The complete process ensures that when the cell divides, each new daughter cell will receive an exact copy of chromosomes, contributing to proper cell functionality and genetic information passage.