What fiber is used in fiber-optic cabling to protect the inner core and prevent the cable from stretching?

Answer: Kevlar

Explanation:

Kevlar is a strong synthetic polymeric fiber with high heat resistance property. Kevlar is used in bullet proof vest, bicycle tires, racing sails, etc, this is as a result of it high tensile strength to weight ratio which accounts for it being 5 times tougher, durable and stronger than steel. In the 1970's kevlar was first used on a commercial scale.