Final answer:
The best way for a playwright to economically adapt source material for the stage is by cutting or simplifying characters and scenes.
Step-by-step explanation:
The best way for a playwright to economically adapt source material for the stage is by cutting or simplifying characters and scenes. By reducing the number of characters and scenes, the production costs can be minimized. This allows for a smaller cast, simpler set design, and fewer technical requirements. Additionally, sticking closely to the original source material can help maintain the integrity of the story while still making necessary adaptations.