Answer: A scrim is a thin backdrop curtain.
Step-by-step explanation: The word "scrim" is a noun and it is used mainly to refer to a woven or cotton fabric that is used to design a curtain. Moreover, "scrim" is generally used for theatre curtains; however, it can also be used for making clothing. Taking this into consideration, the option that fills in the blank is "a thin backdrop curtain". In contrast, "a spinning stage", "a literary philosophy" and "another term for playwright" are definitions that belong to other terms.