A.
anwered by: canteau
The 11-speed chain should fit on the chainrings, the issue is going to be the spacing between the chainrings on a 9-speed crank is much wider than what is on an 11-speed.
The other issue will be with the front derailleur. Shimano's XT M8000 triple front derailleur works with a cog with a max size of 40t. To increase tire clearance, modern front mountain derailleurs have short cage lengths which results in a lower max tooth range. The cage is also narrower on an 11-speed front derailleur to work with the narrower 11-speed chain.