Final answer:
The correct answer to the question about what occurs when a double-stranded break happens during G1 is Non-homologous end joining (B). This process repairs double-stranded DNA breaks by directly joining the broken ends.
Step-by-step explanation:
When a double-stranded break occurs during G1, it prevents the completion of DNA replication. The correct answer to the question posed is Non-homologous end joining (B). This is a repair mechanism that deals with double-stranded breaks before DNA has replicated, which is the stage in G1.
Non-homologous end joining operates by directly joining the two ends of the damaged DNA, often resulting in deletion of a few nucleotides at the breakage site. Homologous recombination (A), in contrast, typically requires the presence of a sister chromatid as a template for accurate repair, which is not available during G1.
The G1 checkpoint (C) is a control mechanism that can arrest the cell cycle if DNA damage is detected but does not directly address the repair of double-stranded breaks. Lastly, DNA ligase malfunction (D) refers to a defect in the enzyme responsible for sealing nicks in the DNA but does not explain the inability to complete DNA replication due to a double-strand break.