How to say "backup (option)"?
後備(reserve/backup) - opposite of 先發(starting) / 正選(primary)

副選/次選 (secondary option/ secondary selection) opposite of 首選 (first choice/ first option) or 正選 (primary)

e.g. 


北京大學是我的首選, 天津大學是後備 (Peking University is my first choice, Tianjin University is the backup)
北京大學是我的首選, 天津大學是我副選/ 次選 (Peking University is my first choice, Tianjin University is my secondary option)
先發球員 (starter)- 後備球員 (bench player)
正選球員 (starter)- 副選球員 (second string player)