By how much should a casing be wider than the elastic that is threaded through it?

The correct choice indicates that a casing should be one-fourth inch wider than the elastic threaded through it. This measurement allows for a snug fit of the elastic while ensuring that the fabric can comfortably encase it. When a casing is too tight, it may restrict the elastic’s ability to stretch and function effectively. A quarter-inch difference is typically sufficient to provide ease of movement and flexibility, contributing to the final garment's comfort and fit.

Considering the options, one inch is too wide, resulting in a loose casing that may cause the elastic to shift, while half an inch could also risk excess space. One-third inch is generally more than needed to accommodate the elastic’s fullness and could potentially create gathering that makes the casing bulky. Hence, the one-fourth inch measurement strikes the right balance for effective casing design.