The default slide size in powerpoint 2016 is

Widescreen (16:9)

Explanation:

Widescreen (16:9) is the default slide size in Microsoft PowerPoint 2013 and 2016. Widescreen (16:9) slides have a size of 13.33 x 7.5 inches or 33.867 x 19.05 cm. Standard (4:3) was the default slide size in older versions of Microsoft PowerPoint, like 2003 or 2007

you can only change it if your real name or personal information was in it. and if so you would need to contact them directly with a username change request.

the answer is d. splog