Semi-Conservative DNA Replication Proportions Steps:

• Start with all heavy-heavy (HH) DNA molecules in 15N medium.
• Switch to 14N: first generation replication yields 100% hybrid (HL) molecules, with 50% heavy strands overall.
• Second generation: each HL replicates to one HL and one LL, yielding 50% hybrid molecules and 25% heavy strands (original heavy strands conserved, diluted by new light strands; total strands double).
• Sample from second generation reflects these proportions in the population.

Why B is correct:

• Matches if misinterpreting second generation as 25% hybrid molecules from incorrect dilution count, with 50% heavy strands carried over from first generation hybrids.

Why the others are wrong:

• A: 25% heavy strands correct, but 25% hybrid underestimates (actual 50%).
• C: 50% heavy strands overestimates (actual 25%), 50% hybrid correct.
• D: Identical to C, same errors.

Not enough information to reconcile options with standard semi-conservative model.

Final answer: B