Which of the following describes a closed tunnel that holds elastic?

A casing refers to a closed tunnel of fabric that is designed to hold elastic or drawstrings, allowing for a gathered or fitted effect in clothing. This technique is commonly used in garments like waistbands, cuffs, and necklines, where a more fitted shape is desired. The casing encloses the elastic, providing a smooth finish while also allowing the elastic to stretch and providing the necessary tension.

The other options do not accurately represent this concept. Seam allowance refers to the area between the edge of the fabric and the line of stitching, which allows for construction and adjustments. Bias tape is a strip of fabric cut on the bias, typically used for finishing edges or adding decorative elements. A faux hem is a decorative stitching that gives the appearance of a hem without actually being hemmed, usually seen in casual clothing.