The preposition ‘among’ is used to talk about people or things as part of a group. The preposition ‘among’ is used correctly in the option ‘They are among the most beautiful islands in the world.’ as it talks as if the most beautiful islands are a group. The option ‘The islands are spread out among the Caribbean Sea and the Atlantic Ocean.’ should use preposition ‘between’ instead as it has two different places ‘the Caribbean Sea’ and ‘the Atlantic Ocean’.