In the Soundscan year (January 1, 2015 - December 31, 2015) 1989 is the top-seller with 1.78 million.
The Billboard Chart year (December 2014 - November 2015) apparently concluded on THIS week's chart. So "25" won't have any appearances in Year-End chart by Billboard in December.
However, she will have the #1 selling album in the Soundscan release in January 2016.
Adele previously was #1 on 2011 & 2012's BB200 Year-End + #1 on 2011 & 2012's Soundscan year-end.
"25" will likely be #1 on 2016's BB200 Year-End + #1 on 2015 & 2016's Soundscan year-end.