The right answer is iron.
Let me explain: During the Civil War, the Confederacy wanted to acquire modern armored ships, therefore, gaining advantage over their adversaries. Somewhere around 1861, the Confederate Congress purchased ships called 'ironclads' from overseas, and a few moths later, they started working on constructing and converting their wooden ships to ironclads, by coating the sides of their ships with iron.
The goal behind this new defense strategy was to make it more difficult for heavy artillery to pierce the ships, thus gaining a new naval advantage over the enemy.
I hope this answer helps! Have a great week :)