It depends on what car you have. For Mustangs with manual transmissions:
3.31s were standard on 2011-2014 (S197) & 2015-2017 (S550) Mustang GTs.
3.73s were an option on 2011-2017 GTs and standard on the Track Pack, BOSS, Shelby, and Performance Pack Cars.
3.73s were standard on 2018+ Mustangs.