Final answer:
Metaphase is the phase of mitosis where the chromatid pairs line up in a single file along the imaginary line called the metaphase plate. Each pair of sister chromatids is attached to spindle fibers that extend from opposite poles of the cell.
Step-by-step explanation:
The phase of mitosis during which the chromatid pairs line up in a single file is Metaphase. In Metaphase, the chromosomes align along the metaphase plate, which is an imaginary line in the middle of the cell. Each pair of sister chromatids is attached to spindle fibers that extend from opposite poles of the cell. This alignment ensures that each daughter cell will receive an identical set of chromosomes during cell division.