Final answer:
Nylon is the fiber with a triangular cross-section, a synthetic material used in various products due to its strength and versatility.
Step-by-step explanation:
The fiber that has a triangular cross-section is nylon. Nylon is a synthetic fiber known for its strength and durability. It was developed to replace silk in various applications due to shortages during World War II. Nylon fibers are used in a wide array of products, including clothing, parachutes, and ropes. They can be molded into different shapes for use in electrical equipment, gears, bearings, and valves. Unlike the other fibers mentioned—wool, silk, and cotton—nylon has a distinctive synthetic triangular fiber structure.