Raw natural rubber is really just a tangle of long, loose polymer chains that can slide past each other quite freely, which is why unvulcanized rubber is soft, sticky and loses its shape easily under load. Vulcanization heats this rubber together with sulfur, and the sulfur atoms form short bridges that tie neighbouring chains to each other at scattered points along their length, a bit like stitching loose threads together at intervals so they cannot slip past one another anymore. Once these crosslinks are in place, stretching the rubber still lets the chains uncoil and extend, but the sulfur bridges pull everything back into shape the moment the load is removed, which is what gives vulcanized rubber both its strength and its springy elastic recovery. Without these links the material would simply deform permanently rather than snap back. So the correct choice is option (D).