Sega Master System / Mark III / Game Gear
Either finish the game once, or enter at the title screen. A sound will confirm the code. An Extra Options item will appear on the main menu, where it is possible play in any stage and, in Versus mode, you can play with any character.
In Versus mode, select your character with held. Now, you can select your opponent, instead of the random selection.
See the main page for Mighty Morphin Power Rangers