The last one.
Create a bibliography so you can make sure you've cited sources correctly.
Answer:
The appositive phrase in this sentence is the one between commas <em>"the messiest room in the house"</em>
Explanation:
An appositive phrase provides more information, renames or identifies the previous noun or noun phrase. In this case it identifies <em>Gordon´s room</em> as the <em>messiest room.</em>
