During which phase of mitosis do the chromatids become chromosomes?

Anaphase

Explanation:

In the anaphase stage, the chromatids separate and produce chromosomes

Chromatids consist of two chromosomes and during anaphase these two chromosomes separate from each other to form two independent chromosomes.

The mitotic spindle which is a long protein microtubule separates the chromosomes

Anaphase is the fourth stage in a cell division and it assures that each daughter cell receives equal number of chromosomes.