Diploid nature maintains two copies of each unique DNA molecule across stages

Steps:

• Eukaryotic somatic cells are diploid, with two homologous chromosomes (copies) for each unique DNA molecule type.
• At interphase start (G1), the cell has two unreplicated copies per DNA molecule.
• DNA replicates in S phase, producing sister chromatids, but the two homologous copies persist into prophase start.
• During mitosis, homologs separate in anaphase; at cytokinesis start, the dividing cell contains two sets, each with one copy per homolog, totaling two copies overall per type.

Why D is correct:

• Diploid cells consistently have two copies of each different DNA molecule (homologs) at the start of interphase, prophase, and cytokinesis, per standard cell biology definitions.

Why the others are wrong:

• A assumes one copy, ignoring diploidy.
• B suggests reduction to one in interphase and cytokinesis, contradicting constant homolog pairs.
• C implies increase to two in interphase and cytokinesis but reduction in prophase, which mismatches replication timing.

Final answer: D