Final answer:
Double-strand breaks in DNA can be repaired by homologous recombination repair and nonhomologous end joining. Nucleotide excision repair is not involved in repairing double-strand breaks but is responsible for repairing large structural DNA damage, like thymine dimers.
Step-by-step explanation:
Double-strand breaks in DNA can be repaired by several mechanisms, chief among which are homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). Homologous recombination repairs a double-stranded DNA break using a homologous sequence as a template, relying on proteins such as MRX, MRN, Sae2, and Sgs1. Non-homologous end joining, on the other hand, does not require a homologous template and typically involves the direct joining of the broken DNA ends, which may result in the loss of nucleotides at the break site. Nucleotide excision repair (NER) is involved in repairing bulky lesions like thymine dimers or larger structural changes in the DNA, rather than double-strand breaks.
Therefore, the answer to the question of how double-strand breaks can be repaired includes both homologous recombination repair and nonhomologous end joining, making option D (both a and b) the correct answer.