The book should discuss about the concept of "island of isolation" related to the Garbage Collector - this is not very well described in the content, although it's tested in Review Question #21.
While we don't use the term "island of isolation" we do discuss tracing eligibility and "the object no longer has any references pointing to it" on page 62. That is the concept the review question is testing.
Do keep in mind this is a certification study guide not a full tutorial on Java. We focus on preparing you for the exam.