(game design) tuning gameplay ultimately comes down to; a. creating new characters to pull the player into the story b. creating new enemies/ai to provide more challenge to the player c. adjusting values in an objects parameters or revising scripts to be more efficient or effective d. all of the above.

I’d say B: creating new enemies/AI to provide more challenge to the player

Tuning difficulty in a somewhat quick and dirty way changes the game’s play. It is called tuning for tough. The reward of a gamer in choosing a high difficulty is a feeling of accomplishment and worth. Many developers should factor into these decisions