Answer:
Jack
Explanation:
Biological organisms tend to undergo anatomical changes as they transition into their adult stage. In the next 5 years, as Jack transitions into adulthood, he will likely grow taller and stronger, and will undergo neurological and hormonal changes (changes of the brain and body chemistry).
Overall, Sam can be said to be the one to "change the most" during the next 5 years when compared to someone who's is already an adult, or something inorganic and somewhat durable like a signpost.
