I believe it’s c, if not then a.
1. Would you like to join my band? - add 'you like to'
2. Martina's band has played in concerts since 2013. - add 'since'
3. Her old band was not as good as her new one. - add 'as good'
4. There are six people in Martina's band. - add 'are six people'
5. You don't have to pay anything to get into their concerts. - add 'have to'
I would have to say it's part of the theme.
Morality and politics is the ethos of the story, aka origin aka theme.
With-out a theme there would be no plot...and no setting.
So...
C. Theme
