Question

Read the sentence.

Answer 1

An appositive phrase modifies the subject of a sentence and is usually separated from the rest of the sentence using commas. 
In this sentence, the appositive is one of Jupiter's moons.

Answer 2

Answer:

The correct answer to the question: Which word, or phrase, is an appositive in the sentence: "Io, one of Jupiter´s moons, is dotted with active volcanoes", is, the last option: One of Jupiter´s moons.

Explanation:

The reason for this answer comes from understanding apposition, and appositive words, or phrases, and their purpose. An apposition is usually a noun, or noun phrase, that clarify, or give further information, about the subject of a sentence. Usually, this type of words, or sentence, goes within commas, emphasizing the use of the apposition as a means of clarification for the subject with which the sentence was initiated and in essence, if this apposition is taken out of the sentence, it does not alter the general information of the sentence as a whole. In this case "one of Jupiter´s moons", is an apposition sentence because it is further explaining the subject, which is Io, and if taken out of the sentence, this lack of information would not really alter the general idea of the sentence itself.