Adults 65 and older. Though it is commonly thought that people of this age group are greater victimized, it is actually the opposite!
Another View: Actuallly . . . the question cannot be answered in its present form as different age groups fall victim to different types of crime.
The question must be narrowed down to WHICH type of crime they are referring to.
Rape.
In the US, White grandparents were found to be the least likely racial group responsible for their grandchildren.
The group least likely to be in combination with the White group was the Black group. Historically, there have been significant social, economic, and political barriers that have separated the White and Black communities in various contexts.
The racial group that is least likely to have health insurance is Hispanics. Many Hispanics in the United States may not be legal and have no means to get insurance.
arab
d.transients
People who have lost faith in the process and believe it does not make much difference who is elected are least likely to vote.
minerals
your legs
gen x
Since statistics can't be compiled if the crimes are not reported, there isn't a way to know definitively what the most unreported crime is or which is least likely to lead to an arrest. Any crime that goes unreported is not likely to lead to an arrest. I would suggest that the most serious of those are assaults, especially domestic incidents and child abuse, and even some homicides that result from it. If a crime is reported and the probable cause exists, then an arrest will be made.
Noble gases, such as helium and neon, would be least likely to occur in an organic functional group since they do not typically form chemical bonds with other elements.