after chromosomes are formed, during which phase of mitosis do the chromosomes attach to the spindle fibers…

Brief Explanations:

In mitosis, during metaphase, chromosomes align at the cell - equator and attach to the spindle fibers by their centromeres. Anaphase is when sister chromatids separate, prophase is when chromosomes condense and spindle fibers form, and telophase is the final stage of mitosis when the nuclear envelope reforms.

Answer:

C. metaphase