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

one of Jupiter’s moons

Appositive is noun or noun phrase in which retitles, renames and helps describe the noun that is adjacent to it in the sentence. The appositive is like a more clear illustration or wants to illuminate the said subject in the statement. 
1.          The appositive word in the sentence is the author. Which describes and renames Gary Jackson in the sentence  
2.           It renames Charles Dickens.