Because a group that steers itself based on ancestry, culture, and such, are basing their lives upon their morals and 'ethnicity'. The word 'ethnic' comes from the word 'ethnicity', which basically means culture or morals.
It wouldn't be minority or religious group. A religious group would be something like 'Christian Community'. A clan is too general. You have tons of clans and a clan can be anything (take 'Clash of Clans' for instance).
It's not A because a clan is like a group of friends basically, it's not a minority because that's describing a group of people and not the actual group itself like gays are a minority, and it's not religious because religious groups are based on weather your a catholic, Jewish, Muslim, etc
