Which honorary office must German citizens take on when they are asked to do so?

EDITORIAL LEARNING AID

Why is this answer correct?

Eligible voters can be obliged by the municipality to serve as election volunteers at elections; this honorary office is necessary for conducting the election. Club coaches (a), library supervisors (c) and teachers (d) are not offices to which the state can oblige citizens.

Election volunteer: an honorary office you can be obliged to take on.

This translation and explanation are editorial learning aids. The official German wording and answer key govern the examination.