Section: Word Knowledge

8) Fortunately, they had an auxiliary police force.

1. experienced
2. standard
3. backup
4. armed

The Latin root of the word auxiliary is aux which means augmented or increased.  This makes it easy to see that choice backup is the clear choice as someone that is auxiliary is a backup and not a standard or experienced police force. Although the backup police force may be armed, that is an assumption and not the meaning of auxiliary.