Choose the analigy with the same relationship as downside is to upside A big is large B outside is to inside C boy is to girl D unique is to generic

Upside and downside can mean a positive and a negative aspect of something, respectively. However, none of the options here present a couple of something negative and something positive: so I think we need to use the local meaning. So our analogy is:

on one extreme of some location (on the upper side) : on the other extreme.

Therefore the best other couple is outside and inside: they also show two opposite locations.