Your question is incomplete because you have not provided the underlined phrases, which are:
The biggest industry <u>of the Cook Islands</u> is tourism, which brings in more revenue than exporting fish and fruit.
The Cook Islands is a group <u>of 15 small volcanic islands</u>.
The Cook Islands stretch <u>over 800 miles</u>.
An airport <u>on Rarotonga</u> is a destination for tourists to the Cook Islands.
Answer:
The Cook Islands stretch <u>over 800 miles</u>.
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.
To make sure all aspects of the instruction manual is thorough and effective.
A statement of the author’s purpose in writing the book, which may include suggestions for using the book
Scientists had begun to debate privately whether or not Pluto is a plant, but this was the first time the concept was presented in a museum for all to see
