The significance of the battle between the Monitor and the Merrimack (also known as the CSS Virginia) primarily lies in option b: "ships made from just wood were now obsolete."
This battle, fought on March 9, 1862, marked a pivotal moment in naval warfare, as it was the first meeting in combat of ironclad warships. The introduction of iron armor significantly changed naval strategy and ship construction practices, signaling the end of the era of wooden warships. While the battle itself ended in a stalemate, its implications for naval technology and future warfare were profound.