Which of the following works has Santiago as its protagonist?

The correct option is The old man and the sea.

To determine the correct answer, we can identify the protagonist of each of the literary works listed in the options:
1. The Old Man and the Sea: This is a classic novella written by Ernest Hemingway, published in 1952. The protagonist of the story is Santiago, an aging Cuban fisherman who goes eighty-four days without catching a fish and then embarks on an epic struggle with a giant marlin in the Gulf Stream.
2. Mansfield Park: This novel by Jane Austen focuses on the life of its protagonist, Fanny Price, who is sent to live with her wealthy relatives at their estate.
3. The Mayor of Casterbridge: This novel by Thomas Hardy follows the life, downfall, and death of its tragic protagonist, Michael Henchard.
4. The Lord of the Rings: This epic high-fantasy novel by J.R.R. Tolkien features Frodo Baggins as the primary protagonist tasked with destroying the One Ring.

Therefore, Santiago is the protagonist of The Old Man and the Sea.