The best response to the question about the similarity among the three groups targeted for genocide would depend on the specific groups discussed in your lesson. However, it is common for groups targeted for genocide to share certain characteristics, such as being religious or ethnic minorities.
Given the available options, if the lesson discusses groups that were indeed typically recognized as religious minorities (such as the Holocaust targeting Jews, the Armenian genocide targeting Christians, etc.), then the correct answer would be:
All were religious minorities.
If this does not fit with the context of your lesson, please consider the specific groups mentioned for a more accurate answer.