Without having the background information that the question references to, I would say that the CORRECT ANSWER IS B.
Epigenetics is the study of physiologic and cellular changes not caused by DNA sequence changes. Using twins in epigentic studies because it allows researchers to estimate phenotypic heritability. Because identical (monoygotic) twins have the same DNA, they are helpful in studying epigenetic markings. Epigenetic markings are affected by environmental factors. Studying twins who share identical genetic composure allow researchers to zero in on environmental differences.