What type of light filter is used to achieve various lighting effects in theatre?

The correct answer is gel, which is a type of colored filter used in theatrical lighting to modify the color of the light emitted from a lamp. Gels are essential for creating different moods and atmospheres on stage by altering the color temperature of lights, enhancing the visual storytelling of a performance. When placed in front of a light source, gels can create a wide range of colors, allowing designers to match specific artistic visions or to reflect different times of day and emotional tones within a scene.

Other options, while related to lighting, serve different functions. Diffusers are used to soften and spread light, helping to reduce harsh shadows. Scrims are often used to create transparency effects and can change how light interacts with various set pieces but do not modify color. Barn doors are an accessory that helps control the direction and shape of the light beam but do not affect the color.