Final answer:
The most commonly shared trait among adults who abuse children is a complex interplay of multiple risk factors including their own history of being abused, substance abuse problems, social isolation, depression, low parental education, and being young parents themselves.
Step-by-step explanation:
Among adults who abuse children, the trait most commonly shared by all of them is not a singular characteristic but a range of potential risk factors, which can contribute to the likelihood of child abuse. While it's true that many adults who abuse children were themselves abused as children, this factor does not alone determine whether an individual will become an abuser. Addiction to drugs and alcohol, a history of mistreatment as a child, low parental education, depression, social isolation, and young or teenage parenthood are all risk factors associated with a higher risk of child abuse. Approximately 30 percent of children who are abused will later abuse their own children, but this also means the majority of abused children do not go on to abuse their own children. Therefore, while answer (a) 'They were themselves abused as children' is a factor, it is not the sole trait and it's important to consider the complex interaction of multiple risk factors.