Stage X is anaphase in mitosis

Steps:

• Recall mitosis sequence: prophase (chromosomes condense, nuclear envelope breaks), metaphase (chromosomes align at equator with spindle attachment), anaphase (chromosomes separate), telophase (nuclei reform).
• Match choices to stages: B describes prophase, A and C describe metaphase (C incorrectly implies meiosis pairing), D describes separation.
• Infer X as the missing separation step after alignment, before reformation.
• Confirm D fits anaphase mechanics.

Why D is correct:

• In anaphase, microtubules depolymerize and shorten, contracting spindle fibers to split centromeres and pull sister chromatids to opposite poles, ensuring equal distribution.

Why the others are wrong:

• A: Describes metaphase, where spindles attach and chromosomes align singly, not separate.
• B: Describes prophase, early condensation and envelope breakdown, before alignment.
• C: Describes metaphase alignment but "in pairs" refers to meiosis I synapsis, not mitosis.

Final answer: D