___________ is a huge curtain made of fire resistant material that quickly seals off the entire proscenium opening in case of a fire on stage

A fire curtain is specifically designed for the safety of theatrical performances. It is a large, heavy curtain made of fire-resistant material that serves a crucial role in fire safety by quickly sealing off the stage from the audience area in the event of a fire. This quick deployment helps to contain the flames and smoke, allowing for a safer evacuation and reducing the potential for fire to spread to the audience.

The grand drape is primarily used for aesthetic purposes to mask the stage from the audience before a performance begins and may not have fire-resistant properties. The act curtain, also known as a show curtain, is used to separate different acts of a performance, but it doesn't serve the same fire safety function. A scrim is a type of gauzy fabric used for special effects in lighting and scene transitions, rather than for fire containment. Thus, the fire curtain's specific design and functionality for fire safety make it the correct answer.