What fabric is used to mask lighting rigs or flown scenery from the audience?

The correct choice refers to "border" fabric, which is specifically designed for masking or concealing elements of a theatre's technical setup, such as lighting rigs or flown scenery, from the audience's view. They are typically made from opaque materials that effectively block light and prevent distractions, allowing the audience to focus on the performance. Borders are usually hung above the stage and can also contribute to the overall aesthetic of the stage design by framing the performance area.

In other contexts, such as with 'blacks', these would also be used for masking; however, 'border' specifically relates to the horizontal pieces that create the top masking effect. 'Burnout' refers to a fabric technique where a pattern is created by burning away parts of a fabric, which does not have masking properties. 'Breakaway' fabrics are designed to separate easily, often used for special effects during a performance, which is also unrelated to the act of masking technical apparatus from the audience. Each of these other options plays a role in theatre practice but does not serve the specific purpose of masking that borders do.