Which of the following is NOT classified as a synthetic fiber?

Rayon is the correct answer because it is classified as a semi-synthetic fiber, not a fully synthetic fiber. Semi-synthetic fibers are made from natural materials that are chemically processed to produce fibers, while synthetic fibers are completely man-made from petrochemical sources.

In contrast, the other fibers listed—polypropylene, nylon, and polyester—are fully synthetic, as they are derived from synthetic polymers created through chemical processes involving petroleum or other resources. This distinction is essential in understanding the classification of textiles and their respective properties. Rayon, being derived from natural cellulose (commonly from wood pulp), fits the criteria for semi-synthetic, whereas the others do not, solidifying the classification of rayon as the exception in this list.