Final answer:
Linen, made from flax plant fibers, is the fiber that is stronger than cotton and increases in strength when it becomes wet.
Step-by-step explanation:
The fiber that is stronger than cotton and increases in strength when wet is linen. Linen is made from flax plant fibers and is well known for its high absorbency and durability. It is recognized for becoming stronger when wet, which is a rather unique characteristic among textile fibers. In contrast, nylon does not increase in strength when wet; it is a synthetic fiber known for its toughness, high tensile strength, and resistance to abrasion, but its tensile strength actually decreases slightly when it absorbs moisture. Wool is a natural fiber that retains warmth even when wet, however, it does not become stronger when wet. Silk, with its luxurious texture and strength, also does not increase in strength when wet. Therefore, linen is the correct answer from the choices given.