Io, one of jupiter’s moons, is dotted with active volcanoes. which word or phrase is an appositive in the sentence?

The answer to your question would be that hte appositive phrase in this sentence is "one of Jupiter's moons".

Explanation:

An appositive is a noun or noun phrase that renames another noun right beside it. Nonessential appositives are always separated from the rest of the sentence by means of commas, as in this example.  As the information they provide is not relevant to complete the meaning of the sentence or to identify the noun they modify, they are offset with commas.

not concrete, existing in thought or as an idea but not having a physical or concrete existence

im not 100 percent sure but i think its a