The correct answer is A: It signifies that Ahab has pitted himself against nature in an effort to dominate it.
Form the beggining of chapter 28, Ahab is portrayed not just as a captain, but as a supreme lord that dictates the destiny of everyone on the ship. That indicates the force of his will. When Ishmael describes the captain over the deck, he observes no signs of illness or weakness. On the contrary, he compares Ahab with Perseus. Moreover, the narrator recalls that Tashego afirmed that the scar on the face of Ahab was the product of a cosmic battle. After describing how the ivory leg was made, Ishmael recollects another comentary from Tashego. There is a paralelism between Ahab and a ship: if a ship loses its mast, it has to be replaced inmediately whithout the need to return home. Similarly, Ahab replaced his leg in order to cotinuo his fight against nature in his rebelion against it, trying to dominate forces that may seem greater that him -but he doesn't believe so.