* 3-speed manual: This was a common base transmission.
* 4-speed manual: A more performance-oriented option.
* 3-speed automatic (Torqueflite): Chrysler's automatic transmission of the era.
To know for sure what transmission is in *a specific* 1976 Dodge D100, you'd need to check the vehicle's documentation (owner's manual, build sheet if available) or look at the transmission itself. The transmission identification number is usually stamped on the transmission case.