Answer: December 31
Explanation:
Granite Slab should adopt the year-end of its principal partners.
A principal partner is one who owns 5% or more of a company's stock.
There are therefore two (2) principal partners here: Nelson Black and Brittany Jones as they own 22% and 24% respectively.
Both of these partners have December 31 as their year end so Granite Slab should adopt this date as well.