Your question is incomplete because you have not provided the underlined phrases, which are:
The biggest industry of the Cook Islands is tourism, which brings in more revenue than exporting fish and fruit.
The Cook Islands is a group of 15 small volcanic islands.
The Cook Islands stretch over 800 miles.
An airport on Rarotonga is a destination for tourists to the Cook Islands.
Answer:
The Cook Islands stretch over 800 miles.
Step-by-step explanation:
Prepositional phrases begin with a preposition and have an object and, sometimes, modifiers. They can also act as adjectives when they modify nouns, and as adverbs when they modify verbs, by demonstrating how, when, where and why the action occurs. In this case, the underlined prepositional phrase functions as an adverbial phrase because it shows where or to what extent the Cook Islands stretch.
The rest of the options are incorrect because they modify nouns, so their object is related to the nouns they modify.