Observing the textures, some fabrics appear to have a uniform color and texture, which can be described as 'plain'.
Other fabrics have visible lines or repeating patterns that suggest a 'striped' or 'checked' design, although the patterns are not very distinct for all of them.
The option 'polka-dot' implies a pattern of dots, which is not clearly visible in any of the fabrics.
Given the options and the visual evidence, 'plain' is a valid description for some of the fabrics, and the selection indicates it is the intended answer.