Which best describes another comparison that melville could have used to symbolize the rigid and unalterable character of ahab in chapter 28 of mobydick?
a.)an uninhabited island
b.) a full moon
c.)a stone monument
d.) a forest fire

The character portrayal of Ahab is scripted as a very reserved person. Especially,  after leaving Nantucket, he is not seen much by his crew on the deck.

From the description in the question, the alternate word that can express the firm and unchanging traits of the protagonist in Chapter 28, in my opinion , must be a static object. It must embody no life or movement in its definition. This kind of similarity would supplement the personality of Ahab. And here, 'a stone monument' can symbolize lifeless-rigidity in behavior which will totally complement Ahab.

Therefore, I think option-"C" is correct.