The correct answer is: "steel".
The second industrial revolution is characterized by the expansion of electricity, petroleum and steel. In opposition, the first industrial revolution had ocurred mostly due to the proliferation of means of transport (railroad, shipping), coal mining, iron industries and textile industries.
During the second industrial revolution, old products and production mechanisms were replaced by the new ones. For example, steel started to replace iron in construction projects, railroads, industrial machines, ships, etc.